Stale NFS Causes BackupPC fileListReceive Failure

Recently, one of my BackupPC clients running CentOS failed to backup, with the contents of the host log being:

    2015-06-10 01:40:10 incr backup started back to 2015-05-16 08:56:42 (backup #600) for directory /
    2015-06-10 21:40:18 Aborting backup up after signal ALRM
    2015-06-10 21:40:18 Got fatal error during xfer (fileListReceive failed)

...and the last bad XferLOG containing:

    fileListReceive() failed

This happened a couple of times in a row, and the interval between the start time of the backup and the failure was consistently 20 hours. While checking, I noticed that an rsync process started on the client by BackupPC was running for about a week. I did an strace -p <PID> on the process ID of rsync and noticed that it was trying to stat an old NFS export, mounted from a server that no longer exists.

Although there are other ways to fix this, it was OK for this host to be rebooted at the time, problem solved.