To follow up on this: the cause really has been a stray jrebel.jar within the tomcat’s lib directory. Other colleagues were afected, too, so I assume that there must be some misbehaving in maven, eclipse or jrebel which put it there.
Anyway: removing jrebel.jar from tomcats’s lib dir made the problem go away, so if you stumble across the same problem, this might be a solution for you, too.