Me again. I managed to increase the gnutls debug level:
dirmngr
dirmngr[1888]: NOTE: this is a development version!
dirmngr[1888]: enabled debug flags: x509 crypto memory cache memstat hashing ipc dns network lookup extprog
dirmngr[1888]: Fehler beim Öffnen von C:\Users\Jan\AppData\Roaming\gnupg\dirmngr_ldapservers.conf': No such file or directory dirmngr[1888]: DBG: number of certs loaded from store 'ROOT': 56 dirmngr[1888]: DBG: Zertifikat
CA’ ist bereits im Zwischenspeicher
dirmngr[1888]: DBG: number of certs loaded from store ‘CA’: 32
dirmngr[1888]: Zertifikat `D:\SwissSign_Silver_CA_-_G2.pem’ ist bereits im Zwischenspeicher
dirmngr[1888]: dauerhaft geladene Zertifikate: 89
dirmngr[1888]: zwischengespeicherte Zertifikate: 0
dirmngr[1888]: vertrauenswürdige Zertifikate: 89 (88,0,0,1)
dirmngr[1888]: DBG: chan_0x00000274 → # Home: C:\Users\Jan\AppData\Roaming\gnupg
Home: C:\Users\Jan\AppData\Roaming\gnupg
dirmngr[1888]: DBG: chan_0x00000274 → # Config: C:\Users\Jan\AppData\Roaming\gnupg\dirmngr.conf
Config: C:\Users\Jan\AppData\Roaming\gnupg\dirmngr.conf
dirmngr[1888]: DBG: chan_0x00000274 → OK Dirmngr 2.2.1-beta9 at your service
OK Dirmngr 2.2.1-beta9 at your service
KEYSERVER --clear hkps://pgp.mailbox.org
dirmngr[1888]: DBG: chan_0x00000274 ← KEYSERVER --clear hkps://pgp.mailbox.org
dirmngr[1888]: DBG: chan_0x00000274 → OK
OK
KS_SEARCH – support@mailbox.org
dirmngr[1888]: DBG: chan_0x00000274 ← KS_SEARCH – support@mailbox.org
dirmngr[1888]: DBG: dns: dnsserver[0] ‘134.34.3.2’
dirmngr[1888]: DBG: dns: dnsserver[1] ‘134.34.3.3’
dirmngr[1888]: DBG: dns: libdns initialized
dirmngr[1888]: DBG: dns: getsrv(_pgpkey-https._tcp.pgp.mailbox.org) → 0 records
dirmngr[1888]: DBG: dns: resolve_dns_name(pgp.mailbox.org): Erfolg
dirmngr[1888]: resolve_dns_addr for ‘pgp.mailbox.org’: ‘pgp.mailbox.org’ [already known]
dirmngr[1888]: DBG: http.c:connect_server: trying name=‘pgp.mailbox.org’ port=443
dirmngr[1888]: DBG: dns: resolve_dns_name(pgp.mailbox.org): Erfolg
dirmngr[1888]: DBG: http.c:1819:socket_new: object 0x03018fb8 for fd 908 created
dirmngr[1888]: DBG: ntbtls(2): handshake
dirmngr[1888]: DBG: ntbtls(2): client state: 0 (hello_request)
dirmngr[1888]: DBG: ntbtls(3): flush output
dirmngr[1888]: DBG: ntbtls(2): client state: 1 (client_hello)
dirmngr[1888]: DBG: ntbtls(3): flush output
dirmngr[1888]: DBG: ntbtls(2): write client_hello
dirmngr[1888]: DBG: ntbtls(3): client_hello, max version: [3:3]
dirmngr[1888]: DBG: ntbtls(3): client_hello, current time: 1505733222
dirmngr[1888]: DBG: client_hello, random bytes: 59bfaa6669d0d44a6df21fd0d5355d668c95f70a2ecf2387c9bdfe4380c08cea
dirmngr[1888]: DBG: ntbtls(3): client_hello, session id len.: 0
dirmngr[1888]: DBG: client_hello, session id:
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49192 TLS-ECDHE-RSA-WITH-AES-256-CBC-SHA384
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 107 TLS-DHE-RSA-WITH-AES-256-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49172 TLS-ECDHE-RSA-WITH-AES-256-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 57 TLS-DHE-RSA-WITH-AES-256-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49271 TLS-ECDHE-RSA-WITH-CAMELLIA-256-CBC-SHA384
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 196 TLS-DHE-RSA-WITH-CAMELLIA-256-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 136 TLS-DHE-RSA-WITH-CAMELLIA-256-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49191 TLS-ECDHE-RSA-WITH-AES-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 103 TLS-DHE-RSA-WITH-AES-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49171 TLS-ECDHE-RSA-WITH-AES-128-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 51 TLS-DHE-RSA-WITH-AES-128-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49270 TLS-ECDHE-RSA-WITH-CAMELLIA-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 190 TLS-DHE-RSA-WITH-CAMELLIA-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 69 TLS-DHE-RSA-WITH-CAMELLIA-128-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49170 TLS-ECDHE-RSA-WITH-3DES-EDE-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 22 TLS-DHE-RSA-WITH-3DES-EDE-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49208 TLS-ECDHE-PSK-WITH-AES-256-CBC-SHA384
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 179 TLS-DHE-PSK-WITH-AES-256-CBC-SHA384
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49206 TLS-ECDHE-PSK-WITH-AES-256-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 145 TLS-DHE-PSK-WITH-AES-256-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49307 TLS-ECDHE-PSK-WITH-CAMELLIA-256-CBC-SHA384
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49303 TLS-DHE-PSK-WITH-CAMELLIA-256-CBC-SHA384
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49207 TLS-ECDHE-PSK-WITH-AES-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 178 TLS-DHE-PSK-WITH-AES-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49205 TLS-ECDHE-PSK-WITH-AES-128-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 144 TLS-DHE-PSK-WITH-AES-128-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49302 TLS-DHE-PSK-WITH-CAMELLIA-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49306 TLS-ECDHE-PSK-WITH-CAMELLIA-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49204 TLS-ECDHE-PSK-WITH-3DES-EDE-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 143 TLS-DHE-PSK-WITH-3DES-EDE-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 61 TLS-RSA-WITH-AES-256-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 53 TLS-RSA-WITH-AES-256-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 192 TLS-RSA-WITH-CAMELLIA-256-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 132 TLS-RSA-WITH-CAMELLIA-256-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 60 TLS-RSA-WITH-AES-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 47 TLS-RSA-WITH-AES-128-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 186 TLS-RSA-WITH-CAMELLIA-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 65 TLS-RSA-WITH-CAMELLIA-128-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 10 TLS-RSA-WITH-3DES-EDE-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 183 TLS-RSA-PSK-WITH-AES-256-CBC-SHA384
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 149 TLS-RSA-PSK-WITH-AES-256-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49305 TLS-RSA-PSK-WITH-CAMELLIA-256-CBC-SHA384
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 182 TLS-RSA-PSK-WITH-AES-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 148 TLS-RSA-PSK-WITH-AES-128-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49304 TLS-RSA-PSK-WITH-CAMELLIA-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 147 TLS-RSA-PSK-WITH-3DES-EDE-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 175 TLS-PSK-WITH-AES-256-CBC-SHA384
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 141 TLS-PSK-WITH-AES-256-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49301 TLS-PSK-WITH-CAMELLIA-256-CBC-SHA384
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 174 TLS-PSK-WITH-AES-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 140 TLS-PSK-WITH-AES-128-CBC-SHA
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 49300 TLS-PSK-WITH-CAMELLIA-128-CBC-SHA256
dirmngr[1888]: DBG: ntbtls(5): client_hello, add ciphersuite: 139 TLS-PSK-WITH-3DES-EDE-CBC-SHA
dirmngr[1888]: DBG: ntbtls(3): client_hello, got 54 ciphersuites
dirmngr[1888]: DBG: ntbtls(3): client_hello, compress len.: 2
dirmngr[1888]: DBG: ntbtls(3): client_hello, compress alg.: 1 0
dirmngr[1888]: DBG: ntbtls(3): client_hello, adding server name extension: ‘pgp.mailbox.org’
dirmngr[1888]: DBG: ntbtls(3): client_hello, adding signature_algorithms extension
dirmngr[1888]: DBG: ntbtls(3): client hello, adding supported_elliptic_curves extension
dirmngr[1888]: DBG: ntbtls(3): client hello, adding supported_point_formats extension
dirmngr[1888]: DBG: ntbtls(3): client_hello, adding session ticket extension
dirmngr[1888]: DBG: ntbtls(3): client_hello, total extension length: 68
dirmngr[1888]: DBG: ntbtls(3): write record
dirmngr[1888]: DBG: ntbtls(3): output record: msgtype = 22, version = [3:3], msglen = 222
dirmngr[1888]: DBG: output record sent to network: 16030300de010000da030359bfaa6669d0d44a6df21fd0d5355d668c95f70a2e
dirmngr[1888]: DBG: cf2387c9bdfe4380c08cea00006c00ffc028006bc0140039c07700c40088c027
dirmngr[1888]: DBG: 0067c0130033c07600be0045c0120016c03800b3c0360091c09bc097c03700b2
dirmngr[1888]: DBG: c0350090c096c09ac034008f003d003500c00084003c002f00ba0041000a00b7
dirmngr[1888]: DBG: 0095c09900b60094c098009300af008dc09500ae008cc094008b020100004400
dirmngr[1888]: DBG: 000014001200000f7067702e6d61696c626f782e6f7267000d00160014060105
dirmngr[1888]: DBG: 0104010301020106030503040303030203000a000400020017000b0002010000
dirmngr[1888]: DBG: 230000
dirmngr[1888]: DBG: ntbtls(3): flush output
dirmngr[1888]: DBG: ntbtls(3): message length: 227, out_left: 227
dirmngr[1888]: DBG: ntbtls(3): es_write returned: success
dirmngr[1888]: DBG: ntbtls(2): client state: 2 (server_hello)
dirmngr[1888]: DBG: ntbtls(3): flush output
dirmngr[1888]: DBG: ntbtls(2): read server_hello
dirmngr[1888]: DBG: ntbtls(3): read record
dirmngr[1888]: DBG: ntbtls(3): fetch input
dirmngr[1888]: DBG: ntbtls(3): in_left: 0, nb_want: 5
dirmngr[1888]: DBG: ntbtls(3): es_read returned: success
dirmngr[1888]: DBG: ntbtls(3): input record: msgtype = 21, version = [3:3], msglen = 2
dirmngr[1888]: DBG: ntbtls(3): fetch input
dirmngr[1888]: DBG: ntbtls(3): in_left: 5, nb_want: 7
dirmngr[1888]: DBG: ntbtls(3): es_read returned: success
dirmngr[1888]: DBG: input record from network: 15030300020228
dirmngr[1888]: DBG: ntbtls(2): got an alert message, type: [2:40]
dirmngr[1888]: DBG: ntbtls(1): is a fatal alert message (msg 40)
dirmngr[1888]: DBG: ntbtls(1): read_record returned: Fatale “Alert” Nachricht erhalten
dirmngr[1888]: DBG: ntbtls(2): handshake ready
dirmngr[1888]: TLS handshake failed: Fatale “Alert” Nachricht erhalten
dirmngr[1888]: Fehler beim Verbinden mit ‘https://pgp.mailbox.org:443’: Fatale “Alert” Nachricht erhalten
dirmngr[1888]: DBG: ntbtls(2): release
dirmngr[1888]: command ‘KS_SEARCH’ failed: Fatale “Alert” Nachricht erhalten
dirmngr[1888]: DBG: chan_0x00000274 → ERR 285212905 Fatale “Alert” Nachricht erhalten
ERR 285212905 Fatale “Alert” Nachricht erhalten
Best,
Jan
P.S.: By the way, it is a Windwows 10 Enterprise Version 10.0.15063 Build 15063 64bit machine.