Thanks for your help, Clarence.
Unfortunately the settings described by fred did not work as intended on my machine.
Only resetting the systems resolution to 100% did the trick. Sadly I would need new glasses for this resolution, so I switched it back to 110% as before.
Fortunately the described displaced buttons are only a small visual flaw and work perfectly although not displayed correctly.
Interestingly enough your "Cassette C90" skins are displayed without any problems. And since I like the C90 skin I simply use this one.
Thanks again, stay healthy in this troubled times and keep on your great skin works.