error signing packages - apk ad hoc

Hey guys
So when I make an apk with archieve and everything and then release it AdHoc an error occurs.
After I put the Password in the certificate to put the apk on the Desktop(or wherever) it Fails.
There isn't popping up any message. the apk simply isn't where it should be. it's nowhere.
the sign on the right is red (normally it's green) and in the error row stands:
Error signing packages. The directory is not empty.
(German:Fehler beim Signieren von Paketen.Das Verzeichnis ist nicht leer. )
In normal release and debug mode everything is working fine.
I work with Visual Studio 2017 and Friday this error occured and I can't find out why it doesn't work...
I didn't Change anything! Really! when i tried it on another PC everything was fine the first time but when I did it again this error occured and it isn't going away.... Even Google couldn't help me until now...
Plz Help!