Kobo WiFi doesn't find any books

My Kobo WiFi doesn't recognize any ebooks at all.

Using Calibre I get the error message below :

unable to open database file
Traceback (most recent call last):
File "site-packages\calibre\gui2\device.py", line 86, in run
File "site-packages\calibre\gui2\device.py", line 490, in _books
File "site-packages\calibre\devices\kobo\driver.py", line 290, in books
OperationalError: unable to open database file

But I can transfer ebooks to the device. Its there but somehow Kobo is not finding them. If I use Kobo desktop the device is not found at all.

I reset (hard and software) the device many times without success.

I am very happy if someone could point me into the right direction.

Thanks and best regards, AcHI