Using Firefox 61.0.2 I get the following error, for this website (wald.intevation.org etc)
Your connection is not secure
The owner of wald.intevation.org has configured their website improperly. To protect your information from being stolen, Firefox has not connected to this website.
wald.intevation.org uses an invalid security certificate.
The certificate is not trusted because the issuer certificate is unknown.
The server might not be sending the appropriate intermediate certificates.
An additional root certificate may need to be imported.
Error code: SEC_ERROR_UNKNOWN_ISSUER