Help - No books in Calibre main storage

I have a strange problem with Calibre.

The reader is recognised, everything seams OK.
If I click on the button "Books on Device" in the main storage, there is no book.

If I add a book from the Library to the device, it is shown in the main storage from my device, and also the green hook in Calibre which indicates that the book is on the device.

If I close Calibre and open it again, there is no sign, that the book is on the device. And also the green hook is away in Calibre.
The main storage is blank,but the book is on the device, and I can read them.

What this is and how I can change this. This has struck me before about 5 updates, however, I had no time to write this here.


Thanks for your help.

Vauh