Thanks, it turned out it was the IPv6 setting of the domain. I thought that adding the IP address in Tools & Settings > IP Addresses was enough, but apparently not
I have a difficult issue with my Plesk managed STRATO VPS and really need help. Whenever I visit the main domain with the OwnCloud site that is installed there, everything is just fine. Correct page, correct certificate. However, some visitors (not all) somehow get to the Plesk login screen and...