I opened a call with RedGate Backup support team and they confirmed they use VDI for their backups, which is also used by Nimble VSS backups. Following a suggestion made by them, I disabled SQL server VSS Writer service, but this effectively "killed" the Nimble VSS backups, they can no longer run.

So it looks like redGate can't coexists nicely with VSS backups? If that's the case, that's actually a big issue.

Is somebody else running VSS backups with RedGate Backup Pro on the same box without any issues?