Hey Boru,
thanks a lot for the suggestions! I went through all your recommended steps to make sure the issue is not on my end.
Cleared my cashes, fixed the disk permissions, reset my preferences, created a new project to ensure no old render files excist, shut off my machine for several times (including unplugging it)...
I used other files this time, just to make sure it is not the files causing it (though they rendered finde without GPU).
And AGAIN issues...
Thank you very much, for your suggestions though!
Anyone else still having problems?