Calibre runs in a loop

Hello,
after playing around a bit there is suddely a new row: "Auf dem Gerät" and Calibre (2.39.0) runs to "Liste der Bücher auf dem Gerät erhalten".
Unfortunately after 50% the program crashes and after restarting it runs again.
I tried to deinstall and install again, but I can't get a virgin program. It just goes on looping.
It says "connected with folder device" but there is no device connected.
No change when I connect a reader.
How can I stop it or how can I completely deinstall the program?
Or where saves Calibre bits that it finds again after reinstall?
Sorry about my english. The program runs in german.