TFS update error: VS402642 found a backup job running against database

Last week, I was performing upgrade to TFS 2015 Update 2. It all looked well for a very long time, untill one of collection databases failed to upgrade on step 1163 of 1171.

Clicking on error gave me some more information:
VS402642 found a backup job running against tfs_xxx database . Wait for the backup job to complete and the rerun the failed collection job from the Status tab.

From TFS Administrator Console i could see that the collection is offline, and job status is Failed. Trying to rerun it immediately fails with the same error.
For the next 15 minutes we tryed to locate if there is anything taking backup of the database, but there was nothing active - and one failed earlier. I could only assume, that backup job on SQL server kicked in while collection was under upgrade and didn't manage to finish, but also blocked successfull TFS update.
Making it short, we had to : quiesce to stop all TFS activities, detach collection database in SQL Management studio (do not detach in TFS!), killing all connections, attach collection database back, unquiesce.
After that job was able to rerun and finish the upgrade.

The lesson learned - make sure no scheduled backups can kick in while you perform your time consuming upgrade to TFS 2015 Update 2.