Mike,
Is there a way to increase the debug reporting level to get more clues?
Les' suggestions are a good way to see if it is a problem on the source file system.
For BackupPC, first try to limit the backup to the smallest case that shows the problem (by excluding files or creating a simple test tree to back up).
Then set $Conf{XferLogLevel} to 9. You should get a lot of output in the XferLOG file.
You then have several options, eg: (1) send the XferLOG file to the list if it short, or to me if it is large - first make sure it doesn't contain anything confidential like file names, host names, IP addresses etc (2) try running manually the tar command (that generates the archive) that is shown in the top of the XferLOG file, and pipe the output to "tar -tvf -". Does it run successfully? Do you see the full path?
Craig