211: KeyNotFound (error code 211)

What does it mean?

An expected key/value was not found. Usually indicates an inconsistency related to internal key management, session keys or the oplog/keys collection.

Why does it happen?

Missing/deleted signing keys in the admin.system.keys collection, clock synchronization problems in the replica set/sharded cluster, keyfile/key rotation problems, or corrupted internal state.

How to fix it

Verify clock synchronization (NTP) between replica set members. Check the contents of admin.system.keys and key generation. Review cluster status (rs.status(), sh.status()). If it persists, analyze the mongod logs and review the keyfile/identity configuration. Make sure the primary is running healthily so the keys can be regenerated.