Xbox App: Cannot read or send messages

I have this issue for several months now. I'm not able to read or send messages through any Xbox app. I'm getting the message: "Please try again. Your messages couldn't be loaded." (message is in German). Xbox support has been ignoring me for 3 months (!)
now. I tried a million things and nothing worked. I have read dozens of MS Answers threads, reddit posts etc - there is absolutely nothing that helped. I can only read and send messages through Xbox and browser (messages are not in chronological order on the
web though, very strange). I cannot read or send messages on: Xbox App + Xbox Beta App for Windows 10 PC, Xbox App + Xbox Beta App for Windows 10 Mobile, Xbox App for Android. Messages do work on the Smartglass app though on the same devices.

Some of the things that I tried:

- re-installing apps

- resetting apps

- logging in and out of profiles

- logging into other profiles (same issue!?)

- deleting old conversations on xbox/browser

- windows app repair / troubleshoot

- signing out of all devices + hard resetting xbox console

- logging into the MS reputation site (that doesn't seem to work at all btw)

- clean start-up

- installing latest windows updates

- removing xbox from xbox preview programme

- ...

Can someone please, please help me out?

:)