Talk for Android is extremely slow on certain mobile or WiFi networks

I’m hosting a Nextcloud instance on my own Nextcloud server (Docker). At the moment version is 29.0.6.
I mainly use Talk and some file sharing. However, the main use is the Talk app for Android.
For a few days now, the Talk app for Android doesn’t work from mobile networks or WiFi networks. On some WiFi networks it is working some other not. I wasn’t able to find the root cause or make the error traceable.
This applies to all of my users. However, I can easily log into my instance and use Talk via the browser.
I am a bit confused because everything worked before.
Any ideas would be much appreciated.

Update: Talk for Android seems to work but is extremely slow. Loading the chat overview or a single chat takes up to 60 sec.
Again, this happens not in all network environments and still, everything is working when I access Nextcloud Talk via Browser.

Regards
Dieter

@didiluca

Nextcloud implemented in Hub 9 the offline-support-function for Nextcloud Talk Android

See https://github.com/nextcloud/talk-android/pull/3952 or Be connected with Nextcloud Hub 9 - Nextcloud for more informations