On Raspberry Calibre ejects Kindle after sending one book

Hello,

I'm using Calibre 3.35 on Raspberry and after I send one book it ejects the Kindle. The device is still mounted but I can't send more books nor open any folder in the file manager. The light on the Kindle is still on but the screen shows the book covers instead of the usb image. I have to disconnect and connect again to send another book.

Any advise?
Thank you