Microsoft recommends creating snapshot backups of no more than 35 content databases at a single time. If more than 35 content databases are backed up as part of a backup process, there is a chance that an AlphaVSSx64 error may appear after the process completes, stating that the SQL Server Writer has been left in a failed state.
Too many content databases are being backed up as part of a backup process. The exact number of content databases that would cause this error varies depending on the server resources and number of worker threads configured on the SQL server.
To resolve this issue, it is recommended that the Max Worker Threads configuration option be increased so that there will be more available threads to work on the snapshot backup. For instructions on how to do so, please click here.
NOTE: This solution will only work for 64-bit versions of SQL server.