[t.below]

I have a similar problem - starting calibre I get this:
calibre, version 2.38.0
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 273, in initialize_db_stage2
File "site-packages\calibre\gui2\main.py", line 213, in start_gui
File "site-packages\calibre\gui2\ui.py", line 362, in initialize
File "site-packages\calibre\gui2\tag_browser\ui.py", line 33, in init_tag_browser_mixin
File "site-packages\calibre\gui2\tag_browser\view.py", line 181, in set_database
File "site-packages\calibre\gui2\tag_browser\model.py", line 282, in set_database
File "site-packages\calibre\db\cache.py", line 57, in call_func_with_lock
File "site-packages\calibre\db\cache.py", line 587, in set_pref
File "site-packages\calibre\db\backend.py", line 126, in set
File "site-packages\calibre\db\backend.py", line 122, in __setitem__
File "site-packages\calibre\db\backend.py", line 806, in execute
File "c:\cygwin64\home\kovid\sw\build\apsw-3.8.2-r1\src\cursor.c", line 231, in resetcursor
CantOpenError: CantOpenError: unable to open database file

Do you have a solution?