Android App can't connect to Nextcloud

so i can access my nextcloud (nextcloudpi: pi4 8gb, nextcloud version, nextcloudpi version 1.46.7) from any device i tried via browser without any problem. The desktop app for windows works fine as well. But the android app doesn’t. Sometimes it works but this is really rare. When i try to connect it says that the host couldn’t be found (my phone is in german, so the error could say something slightly different in english). When my phone is connected to the same network as the nextcloud it works with the local ip of the nextcloud (but it warns me about the wrong ssl certificate). My router is a fritzbox 7590, it is connected via ipv4 as well as ipv6. I use the dyndns service from over my router. I checked the dns records of, the ipv4 and ipv6 are the same that my router has.
I dont know much about this stuff and would be very happy if someone could help me.

Normally the user does not use the internal ip addresses because something does not work. It is normal that you get problems with TLS with internal addresses. For your router you can search Hairpinning and NAT Loopback. Then you can always use from internet and internal network the worldwide DynDNS name with correct TLS.

Does it work from internet (other network) with a PC? Can you test it and post errors?
Post also webserver logs and Nextcloud logs.

It works from my laptop in other networks and i tried it on other pcs as well.
Were do i find the webserver logs?
The Nextcloud logs don’t show anything if i try to connect via the android app. How should i send the logs?

I think NextcloudPi uses Apache2 and then you find the logs in /var/log/apache2 . But i think there are also no logs because the routing from inside to inside does not work. Perhaps there is a setting in your home router to configure Hairpinning or NAT Loopback.

Yes i think there is, i googled it and i can setup an exemption to the DNS-rebind-protection.