In our UAT environment, we have a user that is not able to login. When we look a

So not even other URL worked?
Do you know if this user was renamed at some point? Actually, can you confirm if you have ever used the "Rename Users" plug-in?

Basically if no other URL works this means he should be missing from an engine. Once I hear back from you on these last two questions Appian Support will work with your team directly.

We probably used it on UAT but those users were not used for our test. When I look at the profile of the user, I get the error message, so I can't even deactivate the user.

Ok the fact that you get the same error when you view his profile definitely indicates a problem with the account. At this point makes more sense to work with Appian Support directly.