Numpad and number entry issues in Windows RDP app on MacBook

I have an issue where I am running out of ideas.

My colleague is using a macbook and the Windows app for RDP to connect on a Windows machine.

From one day to the other, when connecting to Windows, we experience keyboard issues:

1. The "+" key on the numpad (external magic keyboard) is not working anymore as expected (it has special function in Datev Tools) -> using the onscreeen keyboard in windows works
2. In certain datev web apps, entering Number is not possible. Copy/Paste from other fields or onscreen keyboard work perfec

We tried fixing the keyboard layout, different (internal and external) keyboards, always same issue. I assume somewhere the mapping is wrong, either in the Windows App or somewhere deep in the Macos system.

using Lates MacOs 26.2 and latest version of the windows app. Anyone got a hint for me? Other users with same setup have no issues...