The "error opening mutex" problem occurs because the semaphore used by SQL Backup has been consumed in SQL Server, but cannot be recovered or recreated.
The solution should be as follows:
Using the tool "Process Explorer" from SysInternals (available from http://www.microsoft.com/technet/sysint ... lorer.mspx
* Click "Find", then "Find Handle or DLL"
* In the box, type "SQBMutex" (without the quotes), and click Search.
This should show the following:
For an instance that is functioning correctly, the handle "\\BaseNamedObjects\\SQBMutex_" and "\\BaseNamedObjects\\SQBMutex_data_" will exist, and be part of the process called "SQBCoreService.exe" (the SQL Backup Agent).
If the instance is not the local one, these names will be appended with the name of the instance (e.g. "SQBMutex_SQL2005")
For the problematic instance, click on the entry in the list (which will likely be linked to "sqlservr.exe" or similar), which will highlight the entry in the main window. Right click this entry and select "Close Handle". Repeat this for the corresponding 'data' handle as well.
You should now be able to start the SQL Backup Agent again.
If that doesn't resolve the situation, please let us know.