The three config servers were migrated to CSRS and WiredTiger last week. Followed the documentation here : https://docs.mongodb.com/manual/tutorial/upgrade-config-servers-to-replica-set/

Since the change, when connection to a mongoS, we cannot see the databases after using show dbs and we cannot connect using --authenticationDatabase admin --username root --password xxxxx --authenticationMechanism SCRAM-SHA-1

There are no errors in any logs, neither mongos, on the config servers, on the mongod machines. rs.conf and rs.status are OK.

The mongos have been stopped and restarted a few times to try and rectify this, with no luck.