Backup job fails: Log reports about 200 of errors but only 1/2 are true

I have a backup job that always backs up data much more slowly than other jobs (about 250MB/min)

When I look in the Job Log, I get a couple hundred errors that say "Directory \\ServerName\Folder1\Folder2\Folder3\etc was not found or could not be accessed." None of these files or subdirectories contained within will be backed up.

Interestingly, when I look at the jobs available for restore, the first 100 errors reported are in fact available for restore!

But the other 1/2 of the errors report file directories that were not backed up.

The file permissions on all these is the same.