Won't Convert - Can't Open Error

Recently Calibre stopped converting files. I am being told it was unable to open database files.

I have updated to most recent version of Calibre, and also restored my library in hopes of solving the problem. Any solutions would be greatly appreciated.

Here is the error code in detail:


calibre, version 3.15.0
ERROR: Unhandled exception: <b>CantOpenError</b>:CantOpenError: unable to open database file

calibre 3.15 embedded-python: True is64bit: True
Darwin-17.3.0-x86_64-i386-64bit Darwin ('64bit', '')
('Darwin', '17.3.0', 'Darwin Kernel Version 17.3.0: Thu Nov 9 18:09:22 PST 2017; root:xnu-4570.31.3~1/RELEASE_X86_64')
Python 2.7.12
OSX: ('10.13.2', ('', '', ''), 'x86_64')
Interface language: None
Traceback (most recent call last):
File "site-packages/calibre/gui2/convert/single.py", line 288, in accept
File "site-packages/calibre/gui2/convert/metadata.py", line 259, in pre_commit_check
File "site-packages/calibre/db/cache.py", line 63, in call_func_with_lock
File "site-packages/calibre/db/cache.py", line 1101, in set_field
File "site-packages/calibre/db/cache.py", line 1036, in mark_as_dirty
File "site-packages/calibre/db/cache.py", line 1029, in update_last_modified
File "site-packages/calibre/db/write.py", line 544, in set_books
File "site-packages/calibre/db/write.py", line 197, in one_one_in_books
File "site-packages/calibre/db/backend.py", line 856, in executemany
File "src/cursor.c", line 236, in resetcursor
CantOpenError: CantOpenError: unable to open database file