SBS Backup Fails on VSS 0x800423f4 error

In the last 2 weeks we've suddenly seen 3 of our client's SBS Servers appear with failed backups and the following in the backup log:

Volume shadow copy creation: Attempt 1."MSDEWriter" has reported an error 0x800423f4. This is part of System State. The backup cannot continue.
Error returned while creating the volume shadow copy:800423f4Aborting Backup.

We're currently looking into it but all of these servers are SBS 2003 SP1 with Exchange SP2 recently installed. I don't know if there is any relation but it sure is more than a coincidence that all of them are starting to have trouble.

Even running the backup now option in the SBS wizard causes the backup to fail as well. We're following with Microsoft to see a resolution and I'll post here when I find it, if I do!