Thank you for your message. Unfortunately, this happens alot. It is indeed not allowed to withhold the deposit without reason/explanation. This is only allowed if you have demonstrably caused damage to the rented property or if utility costs need to be deducted from the deposit. In all cases, the costs must be specified.
Legally, contract costs may indeed not be charged either. In case you wish to take action against this and discuss your case further, please feel free to contact me.