Error in library sync

Hi everyone, I've been using calibre for 3 years now and recently (about 3-4 months ago) I bought a new Kindle Paperwhite Signature Edition. Now, in the beginning everything went fine, but in the last month I started noticing that, while calibre gave me the "kindle recognized" signal, it took longer and longer to actually sync the library. Since yesterday this error started popping up:

C:\r\src\src\calibre\devices\mtp\windows\content_enumeration.cpp:835:Failed to create stream interface to write to object:[hr=0x802a0002 wCode=0 name=E_WPD_DEVICE_NOT_OPEN] Chiamata di arresto già eseguita su questo oggetto o sull'oggetto proprietario.

Traceback (most recent call last):

File "calibre\gui2\device.py", line 110, in run

File "calibre\gui2\device.py", line 581, in _books

File "calibre\devices\mtp\driver.py", line 359, in books

File "calibre\devices\mtp\driver.py", line 400, in write_metadata_cache

File "calibre\devices\mtp\driver.py", line 225, in put_calibre_file

File "calibre\devices\mtp\windows\driver.py", line 37, in check_thread

File "calibre\devices\mtp\windows\driver.py", line 492, in put_file

wpd.WPDError: C:\r\src\src\calibre\devices\mtp\windows\content_enumeration.cpp:835:Failed to create stream interface to write to object:[hr=0x802a0002 wCode=0 name=E_WPD_DEVICE_NOT_OPEN] Chiamata di arresto già eseguita su questo oggetto o sull'oggetto proprietario.

And now it's been 8 mintues just to sync 30% of the library. What should I do? Thanks to anyone that can help!