Sometimes one or more of our backup proceses hang and when this happens the SQL Backup Agent service needs to be restarted in addition to the process being killed on the server. However for some reason (my thesis is that this happens when you do not kill the process on the SQL Server first) sometimes the SQL Backup Agent won't start giving error event 5000.
The following error message is displayed in the event viewer:
SQL Backup Agent startup error: error initialising IPC objects. Error creating mutex (already exists): Global\SQBMutex_<instancename>
Because I have a programming background I know what a mutex is, but if you don't you can find a global description on wikipedia.
My initial thoughts were that SQL Backup Pro was somehow keeping a handle on a mutex or that it didn't clean up properly. So I deinstalled SQL Backup Pro and reinstalled it, but got the same error. I was able to delete all files and there wasn't any SQB process on the server that could have held those handles.
Using Proces Explorer I found out it was actually SQL Server that was keeping the handle on the mutex so in order to be able to start the SQL Backup Agent you have to restart SQL Server.