Clickker uses an Exact user to communicate with the administration in Exact Globe. The username in question is not (anymore) in Exact. This could be caused by changing the username of that particular user in Exact Globe or it has not yet been created in the administration.

Solution: Arrange for that user with that specific username to be in Exact Globe with access rights and preferably the Administrator role.

You can look in the IIS at the application pool (application group) that the Clickker website uses in the advanced settings. There, under Identity, it will be listed which user is using Clickker. Usually, the name of the application pool will simply be "Clickker".