In rdiff-backup's robust.py module, there is a function called catch_error which attempts to "step over" certain operating system errors that should not be fatal to the backup as a whole. In this, EDEADLOCK is listed, but not its BSD equivalent, EDEADLK.

Fix: To fix this, add the EDEADLK error to catch_error in robust.py. EDEADLK could replace EDEADLOCK in the list as FreeBSD doesn't use the latter (that I can tell), but perhaps the patch will make it upstream. I believe tabs have been preserved properly. If not, it's a two-liner anyway. Thanks for your time.
How-To-Repeat: Mount an SMB share via smbfs and use rdiff-backup to back up a directory containing a file that is opened on a Windows machine. Outlook .pst files are the best candidates. rdiff-backup will raise an exception, '[Errno 11] Resource deadlock avoided' upon encountering the file and halt the backup.