After the release of the 21.5 Storage Platform, backups for accounts moved from one Storage Server to another (either manually or automatically) fail to backup.
Affected accounts also cannot be connected to using InstantData.
Accounts that have not moved are not affected.
The 21.5 Storage Platform release introduced a software defect which causes the MoveCopyInProgress flag to be set to "true" even after the move completes.
Ordinarily backup clients will detect this flag, and retry until it clears - but this cannot take place if the flag does not clear.
The setting is cached by the Storage Server, so can only be cleared by updating the flag locally and restarting the Storage Server. This is impractical at scale.
The defect is resolved in Storage Platform version 188.8.131.5211 (21.5 Hotfix 1).