No Qt platform plugin could be initialized


I have been using nextcloud for more than a year on a Windows 10 system. Since a few days ago, the following error message occures when i try to start the nextcloud client:

"This application failed to start because no qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: windows"

What i did:
I have tried repairing the installation as well as uninstalling it and reinstalling nextcloud (with official clients of different versions, 32 and 64bit). Nothing fixed the problem so far.

I have no clue why that problem occurs out of a sudden. If you need more information on my system let me know.