Documented error, it's in the troubleshooting guide
The Backup Server runserver file, the interfaces file, and the sysservers table in the master database do not have a consistent name for the Backup Server.
or, more likely if it's an intermittent error
You have hit a network limit for the Backup Server process. Typically, this limit is set to 25.
Start Backup Server with the -Nnn flag to change the network limit. For example, -N30 increases the limit to 30.