After the release of the 21.5 Storage Platform, backups for accounts that have been moved from one StorageServer to another (either manually or automatically) fail to back up.
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 agents 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 StorageServer, so can only be cleared by updating the flag locally and restarting the StorageServer. This is impractical at scale.
The defect is resolved in Storage Platform version 18.104.22.16811 (21.5 Hotfix 1).