SharePoint backup fails "The transaction log for database "SharePoint_Config" is full" (4035869)

SharePoint backup fails "The transaction log for database "SharePoint_Config" is full"

SharePoint backups start to fail with the below NetVault log messages:

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The transaction log for database 'SharePoint_Config' is full. To find out why space in the log cannot be reused, see the log_reuse_wait_desc column in sys.databases...
STSADM failed
Error running STSADM
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This error is not a NetVault error but is returned by SharePoint regarding its internal SQL database.
Steps outside of the NetVault scope need to be taken to manually shrink the logs.
The source of this error is MSSQLSERVER which indicates that the transaction log(s) associated with a SharePoint content database has grown so large that it has consumed
all available disk space making it impossible for SQL Server to log and maintain itself. 

1) Check the event logs on the server hosting the SQL Server Database backend used by SharePoint.
You should find the same error message reported to NetVault as follows:
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Event Type:       Error
Event Source:    MSSQLSERVER
Event Category: (2)
Event ID:           9002
Date:                 [date]
Time:                 12:07:31 PM
User:                 [domain]\[account]
Computer:         [machine name]
Description:
The transaction log for database 'SharePoint_Config' is full. To find out why space in the log cannot be reused, see the log_reuse_wait_desc column in sys.databases. 
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2) Check the disk containing Content_Config database to confirm the lack of space.

3) Take steps to free up space:
- On your SharePoint SQL Server database, open the SQL Server Management Studio. 
- Connect to the local SQL Server. 
- Right click SharePoint_Config
- Go to -> Properties -> Options. 
- Change Recover Model from Full to Simple -> Click OK. 
- Right click SharePoint_Config, go to Tasks -> Shrink -> Shrink Files. 
- Select a File type of Log, then Shrink.

Following these actions, re-running the failed backup will work.