This recently began to happen when I switched my servers to backup to a mounted NFS export. The backup will run fine, but get hung up at the end and never exit. The next day, I get a failed backup with the error: There are currently 3 backups running, and the system-wide limit is 3. If I send a HUP signal to the hung process, the backup exits.
Now, the interesting part is that I have 2 servers both configured identically. One is off-site and one is local, only one server is having this problem. Guess which one... nope, not the remote one.
Strangely, it's the local server with the low latency high speed link to the backups share that is hanging up.
The remote server is a PowerPC G5, the local one is a VMWare Virtual Machine.