You are protecting Windows Small Business Server 2011 Standard.
When checking VSS doctor report or Windows Application Event log you found the following (or similar) warning for "spsearch account" and 1376 status code:
Message: Volume Shadow Copy Service error: Unable to resolve spsearch account with status 1376. Check connection to domain controller and VssAccessControl registry key. Operation: Collecting Write Process Data Asynchronous Operation Execution Context: Execution Context: Requestor Current Status: GatherWriterMetadata Error Details: Error: NetLocalGroupGetMemebers (spsearch), 0x80070560, The specified local group does not exist.
SBS 2011 Standard Edition installs Sharepoint 2010 Foundation in SharePoint farm mode. The accounts SPfarm and SPsearch are used as service accounts for some of the Sharepoint services. To utilize the VSS writers, the accounts must be granted access to VSS. The accounts are added by SBS to the vssaccesscontrol registry key but the VSS service fails to locate the accounts.
Acronis Agent uses VSS service for application database backups in Windows OS family. As a result, backups start failing.
More info in these Microsoft articles: You may get VSS warnings in the application event log of SBS 2011 and SBS 2011 VSS 8230 errors relating to SharePoint search.
Check if spsearch account exists and has sufficient permissions (part of Administrators group). Follow the MS articles mentioned above.
Should the issue persist, please collect the following logs while experiencing the issue:
and contact Acronis support.