SharePoint cannot connect to SQL Server - Event 3355.

Applies to: SharePoint 2013, SQL Server 2008 R2 Standard Edition.
 

Problem Description.
 

On the Application Log of the Event Viewer of a SharePoint server, we found entries of the Event ID 3355 and they were appearing every day between 9:00 pm and 9:30 pm.  The details of that critical error are the following:
 

Critical Event 3355 SharePoint Foundation
Cannot connect to SQL Server, XXX not found. Additional error information from SQL Server is included below.
Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.

We also found some occurrences of the following related warning on the Application Log.

Warning Event Id 1309 ASP .NET 2.0.50727.0 Source Web Event

Process information:
Process ID: 15292
Process name: w3wp.exe
Account name: domain\SP_Services

Exception information:
Exception type: HttpException
Exception message: Unable to connect to SQL Server session database.

Cause.

We found these errors were related to backups made to the Hyper-V VMs where the SharePoint Servers reside. When the backups were taking place then the event ids mentioned above start to appear, mainly because the Hyper-V VMs were put on status saved as part of the VM backup process.

We also found that the integration services required to perform online backups required an upgrade.

Solution.


Below you will find a documentation on how to perform online backups to Hyper-V VMs that do not affect the availability of virtual machines using System Center DPM.

Back up Hyper-V virtual machines.

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