Calibre and Sony PRS-T1 - Calibre says there are no books on device

[Pippo53s03]

Calibre no longer sees the books contained in my Sony PRS-T1 .
Using Calibre I send epub from my PC to the device; I unplug the device and the books are there; when I reconnect the device, Calibre says that there are no books on device.
Starting Calibre in debug mode I obtain these last messages in log:
...........
DeviceJob: 9 Recupera la lista dei libri del dispositivo callback returned
Job: 9 Recupera la lista dei libri del dispositivo finished
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\prst1\driver.py", line 163, in books
File "site-packages\calibre\devices\usbms\driver.py", line 249, in books
File "site-packages\calibre\devices\prst1\driver.py", line 246, in sync_booklists
File "site-packages\calibre\devices\prst1\driver.py", line 275, in update_device_database
OperationalError: unable to open database file
................
I use win XP s.p.3 and Calibre 1.4.8

please help.

[kovidgoyal]

Do a reset of your device.