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When starting Calibre I get the following error. Once I dismiss the notification Calibre seems to open fine but then Calibre closes unexpectedly when opening a book or searching an author. The little spinning circle in the lower right if the screen indicates a running job but none are shown when clicking that to open the running jobs window. Even though nothing appears, I've tried stopping all non-device jobs but does no help.

calibre, version 4.7.0
ERROR: Startup error: There was an error during calibre startup. Parts of calibre may not function. Click Show details to learn more.

Traceback (most recent call last):
File "site-packages\calibre\gui2\main.py", line 292, in initialize_db_stage2
File "site-packages\calibre\gui2\main.py", line 236, in start_gui
File "site-packages\calibre\gui2\ui.py", line 434, in initialize
File "site-packages\calibre\gui2\init.py", line 781, in save_layout_state
File "site-packages\calibre\gui2\library\views.py", line 636, in save_state
File "site-packages\calibre\gui2\library\views.py", line 630, in write_state
File "site-packages\calibre\db\cache.py", line 62, in call_func_with_lock
File "site-packages\calibre\db\cache.py", line 616, in set_pref
File "site-packages\calibre\db\backend.py", line 134, in set
File "site-packages\calibre\db\backend.py", line 131, in __setitem__
IOError: IOError: disk I/O error