DAV not reachable at all

Following my described problem in


I found out that I cannot connect with any app or sync client anymore, no matter if Windows Sync Client, Android App or DAVdroid. Logging in on the web interface works though, but takes longer than expected. Afterwards it works fine there.

I already read through the forum and found slightly similar scenarios, but none is fitting mine completely!

I am on a HostEurope vServer with Ubuntu 14.04. I am running two domains. Nextcloud is installed in a subdomain of one of them and was running brilliant until today…