epub to azw3 (or mobi) conversion error

I have encountered a problem in attempting to convert the book Our Mathematical Universe by Max Tegmark from an epub file to an azw3 file. This is the detailed error message Calibre throws, it happens immediately:

calibre, version 1.21.0
ERROR: Unhandled exception: <b>IOError</b>:[Errno 13] File is open in another process: u'c:\\documents
and settings\\joseph\\my documents\\my dropbox\\ebook\\calibre\\max tegmark\\our mathematical universe
(1358)\\our mathematical universe - max tegmark.epub'

calibre 1.21 isfrozen: True is64bit: False
Windows-XP-5.1.2600-SP3 Windows ('32bit', 'WindowsPE')
('Windows', 'XP', '5.1.2600')
Python 2.7.5
Windows: ('XP', '5.1.2600', 'SP3', 'Multiprocessor Free')
Traceback (most recent call last):
File "site-packages\calibre\gui2\convert\single.py", line 278, in accept
File "site-packages\calibre\gui2\convert\metadata.py", line 236, in commit
File "site-packages\calibre\db\legacy.py", line 465, in set_metadata
File "site-packages\calibre\db\cache.py", line 57, in call_func_with_lock
File "site-packages\calibre\db\cache.py", line 1153, in set_metadata
File "site-packages\calibre\db\cache.py", line 1140, in set_field
File "site-packages\calibre\db\cache.py", line 944, in set_field
File "site-packages\calibre\db\backend.py", line 1357, in windows_check_if_files_in_use
File "site-packages\calibre\utils\filenames.py", line 370, in __init__
IOError: [Errno 13] File is open in another process: u'c:\\documents and settings\\joseph\\my
documents\\my dropbox\\ebook\\calibre\\max tegmark\\our mathematical universe (1358)\\our mathematical
universe - max tegmark.epub'

I have converted a number of books in the past, both epub → azw3 (or mobi) and azw3 (or mobi) → to epub. This is the first time I have encountered this problem. I can view the book using the Calibre viewer so I don't believe it is a DRM issue. Also I receive the same error message if a attempt a epub → mobi conversion. Also as is apparent from the file path in the error message the epub file in question is synced with my Dropbox account in the cloud. I don't see why this should be an issue as it has never been a problem before. There is no reason any other process should be using the file when I attempt the conversion.

[kovidgoyal]

Shutdown dropbox.

Originally Posted by kovidgoyal

Shutdown dropbox.

Thank you that worked. Something I should have thought of , but I had never encountered that as an issue in prior conversions.