It seems there are at least several problems with training in Monitor. We've had reports of red crosses, the avatar just standing there, walking a few steps and then stopping. In other words, it could use some work.
To be able to properly debug this, I'd like to request some savegames from you. If you have a savegame right before training, that either works properly, or does not work, please e-mail it to me, with a note saying how it behaves exactly when you try to train. (Also mention which party member you tried to train, and with which trainer)
I have a savegame which is sorta locked up while dupre is attempting to train. Would this help? It's actually at the point of the lock up with Dupre attempting to beat down the person he's training with. The person he's training with doesn't seem to care that Dupre is attacking him and keeps going on about the traitor. Dupre doesn't seem to be hurting him at all.