Is is always the same Storrage Groups and if yes is there something unique about the configuration of those mailboxes compared with the ones that work

(DB paths, transaction logs paths, DAG configs etc)

Yes it is the same storage groups all the time

I have tried to backup with OFO / Snapshot fully disabled, same error.

I have used DiskShadow to kill all open / orphaned shadow copies.

Still get the same error.

SG2-Full -- The job failed with the following error: A failure occurred querying the Writer status.

Backup- MSL-SVR169
V-79-57344-65233 - Snapshot Technology: Initialization failure on: "\\MSL-SVR169\Microsoft Information Store\Mailbox_Database_2". Snapshot technology used: Microsoft Volume Shadow Copy Service (VSS).
Snapshot technology error (0xE000FED1): A failure occurred querying the Writer status.
Check the Windows Event Viewer for details.
Writer Name: Exchange Server, Writer ID: {76FE1AC4-15F7-4BCD-987E-8E1ACB462FB7}, Last error: The VSS Writer failed, but the operation can be retried (0x800423f3), State: Waiting for backup complete notification (5).
 

Lee: and your writers are all stable and running after the backup? Normally the way to fix this is to restart the Exchange server, but I have seen some extreme ways to fix problems like this including creating a new Information Store and moving the mailboxes to that...

Thanks!