Backup slows down and eventually fails: [61:1002] The BMA named "" on host reached its inactivity timeout.

For each file that is accessed by VBDA there is a matching event from mvagtsvc.exe for exactly the same file.

Process Monitor logs were filtered to include the events  that take 2 seconds, and we can see this:

9:22:19,4308436 vbda.exe CreateFile \Device\HarddiskVolumeShadowCopy659\Install\7zip\7z1514-x64.exe SUCCESS
9:22:19,4326471 mvagtsvc.exe CreateFile C:\Install\7zip\7z1514-x64.exe SUCCESS
9:22:21,6396038 vbda.exe CreateFile \Device\HarddiskVolumeShadowCopy659\Install\backinfo\Bginfo.exe SUCCESS
9:22:21,6451960 mvagtsvc.exe CreateFile C:\Install\backinfo\Bginfo.exe SUCCESS
9:22:24,9470879 vbda.exe CreateFile \Device\HarddiskVolumeShadowCopy659\Install\symantec\CCS_Agent-11.0-V3-Windows\AssignNewPerm.exe SUCCESS
9:22:24,9687742 mvagtsvc.exe CreateFile C:\Install\symantec\CCS_Agent-11.0-V3-Windows\AssignNewPerm.exe SUCCESS
9:22:27,0559048 vbda.exe CreateFile \Device\HarddiskVolumeShadowCopy659\Install\symantec\CCS_Agent-11.0-V3-Windows\Get_FQDN.exe SUCCESS
9:22:27,0701393 mvagtsvc.exe CreateFile C:\Install\symantec\CCS_Agent-11.0-V3-Windows\Get_FQDN.exe SUCCESS
9:22:29,2168233 vbda.exe CreateFile \Device\HarddiskVolumeShadowCopy659\Install\symantec\CCS_Agent-11.0-V3-Windows\PortQry.exe SUCCESS
9:22:29,2177324 mvagtsvc.exe CreateFile C:\Install\symantec\CCS_Agent-11.0-V3-Windows\PortQry.exe SUCCESS
9:22:31,2212611 vbda.exe CreateFile \Device\HarddiskVolumeShadowCopy659\Install\symantec\CCS_Agent-11.0-V3-Windows\regxp.exe SUCCESS
9:22:31,2291214 mvagtsvc.exe CreateFile C:\Install\symantec\CCS_Agent-11.0-V3-Windows\regxp.exe SUCCESS
9:22:33,2290398 vbda.exe CreateFile \Device\HarddiskVolumeShadowCopy659\Install\symantec\CCS_Agent-11.0-V3-Windows\Setup.exe SUCCESS
9:22:33,2380738 mvagtsvc.exe CreateFile C:\Install\symantec\CCS_Agent-11.0-V3-Windows\Setup.exe SUCCESS

Mcafee AV is creating delays when accessing the filesystems.