The black blocks in Martins picture are relays. They control whether the motors go forward or reverse. They normally only operate for reverse. If a bit of damp has got into the control board the transistors operating the relays might be in a switched on condition, making the motor reverse only. A relay stuck part operated can have the same effect. As they have black covers, you can't see what is going on with the contacts, so there is some guesswork.
Cleaning of both sides of the PCB might remove the unintended circuit that mother nature created. With the battery out, plenty of clean water and some toothbrush action, dry well, a wash over with isopropyl alcohol to dissolve and flush away any lurking hidden drops of water, treat with Corrosion-X.
When reassembling, clogging the wire hole up with something like Blu-Tak will reduce the chance of water getting in that way.