Are both the art.[userID] column and the users.[userID] column number fields? If one is a number field, and one is a text field, you'll get a "type mismatch" error. In other words, the database cannot compare numbers with text characters.
Don't know much about access, but what do you mean that they are "linked" in the database? Also, I vaguely remember something about setting up "expressions" for tables in access (way back when). Have you set any of those up?
Just to let you know by the way, this isn't much of a ColdFusion issue per se, but more of a database issue. You're just querying a database within ColdFusion. That being said, if you run that exact same query within access itself, do you get the same error?