I just wanted to chime in and say I've been having the same exact errors recently. I can add a book to calibre, but anytime I edit the metadata and hit 'ok' I get the above error.
I also read the FAQ when I started and decided to give the NAS storage a shot anyway. I have been keeping my database and back up books on my Synology NAS and it has been working wonderfully for the past year. Then around 9.2 or 9.3 I started getting this error. I understand that network file systems are a different beast but at some point this was working for me.
eg. I add a book by other Bob Smith. -> Calibre creates a folder called Bob Smith with my book, the cover and metadata inside it. If I then edit the metadata for this book to say, Smith, Bob -> Calibre gives me the error - BUT - it creates an empty folder called Smith, Bob while leaving the folder Bob Smith. This seems to be new behavior because it used to work fine.
Should I downgrade a few versions? I really have no use for a library that's not on a NAS.
here's the actual error:
calibre, version 0.9.7
ERROR: Unhandled exception: <b>error</b>
3, 'CreateFile', 'The system cannot find the path specified.')
Traceback (most recent call last):
File "site-packages\calibre\gui2\metadata\single.py", line 472, in accept
File "site-packages\calibre\gui2\metadata\single.py", line 445, in apply_changes
File "site-packages\calibre\gui2\metadata\basic_widgets.py", line 274, in commit
File "site-packages\calibre\library\database2.py", line 2507, in set_authors
File "site-packages\calibre\library\database2.py", line 2494, in windows_check_if_files_in_use
File "site-packages\calibre\utils\filenames.py", line 290, in __init__
error: (3, 'CreateFile', 'The system cannot find the path specified.')