Unable to Decrypt Messages of one specific Matrix Server

[rubo77]

I'm experiencing difficulties decrypting messages on one of our Matrix servers. Here are the details:

1. Scenario:
   • I'm using the Element desktop client and the Android Element App
   • everything worked fine until some months ago when it started, that the user from the matrix.toppoint.de server cannot decrypt my mesages, sent from the Android App while messages sent from my desktop client on Ubuntu can be decrypted just fine.
   • When attempting to verify the QR code with the desktop client again, the QR-Code wasn't accepted.
   • I then used the icons, but encountered the following error message.

in english probably:

One of the following possibilities may be affected:

Your homeserver

The homeserver connected to your counterpart

Your or your counterpart’s internet connection

Your device or your counterpart’s device

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What can I do to debug this further?