6.0 binaries expect that the collection version has a Timestamp, which was introduced as part of the 4.4 FCV -> 5.0 FCV transition in PM-1965. However, it could happen that due  the poor resiliency that DDL operations had in the past there is some garbage config.cache.* metadata that wasn't properly cleaned up and that doesn't contain Timestamps.  This is not a problem until the customer decides to use again that namespace:  then that garbage will be read, trying to understand which is the latest metadata locally persisted on the shard. However, because it doesn't contains a timestamp, it will crash with a unnamed error code, blocking refreshes (routing and filtering) over that namespace.