Well, the message suggests a disk I/O error situation.
Whereas backups more thoroughly traverse file systems than most other
daily operating system activities, it can be expected that the scheduled
backup would be the most common detector of I/O error conditions.
I'd recommend referring to your OS system log (/var/log/messages) for
error indications.  Your client log may indicate where in the file system
the error occurred, by way of progress messages.  In that it looks like
you're having the problem with multiple file systems, there may be a
disk array which is acting up.  You might try running a 'find' command
to traverse the subject file systems to see if the situation pops up
at the OS level.

   Richard Sims