I incidentally came across the solution to my problem by myself, but I will still post this thread as I don't really understand what's going on and already put alot of effort into collecting all neccessary logs. I beg your pardon.
I run the recent version of nextCloud (11) on a RaspberryPI-powered archlinux-arm operation system. The webserver is nginx (1.11.9). Port-forwarding on my router is set from port 4433 to 443 of my server's local IP and I append that port info to DAVdroid accordingly.
I setup everything by the books and could install nextCloud just fine:
- Access via the web interface is possible with a local IP aswell as an external one.
- Import of my self-signed certificate via CAdroid worked perfectly well.
- The nextCloud client for Android is syncing just fine from external IP.
My problem was the following:
- DAVdroid could only connect to my server with a local IP when in the same network. If I tried to connect (to the DNS name of my server) while having an external IP, it failed (calendar and contacts could not be found).
Relevant log files can be found here: https://paste.xinu.at/m-H2fKw/
Now, the solution:
I wanted to test my server with SSLLABS and they can't scan any other port than 443. So, I forwarded that port temporarily to my server in order to be able to test. In this state, I just gave connecting with the external IP another shot and - BAM - it works. Now, could someone tell me what is wrong here? It seems to me like the port-forwarding or redirecting of port 4433 to port 443 does not work. I have, however, port 222 forwarded to 22 for SSH which is working just fine. I would really love to see someone clear the air for me.
Thanks a ton for your time and