This happens when a Client Profile with an active Client Login is deleted. The Client Login that was attached to the profile becomes "orphaned" so when the client logs in they aren't connected to a profile. To fix you need to edit the Client Login from the Administrative > Client Logins page and connect it to a Client Profile. If the profile was deleted due to being a duplicate you will want to merge the profile instead of deleting in the future.

NOTE: The system sends you a "Client Login Misconfiguration" email whenever this happens with directions on how to fix it