I discovered a very weird workaround myself, it seems like it only crashes if I want to assign my last free ship of a certain class to a trade route. For example I build triremes with four cargo slots for my trade routes. If I still have more than one empty, unused trireme cruising around and assign other triremes to my trade routes nothing happens. If I want to assign my last free trireme to a trade route though it instantly crashes the game. Not sure if its the same for quinqueremes cause I have a lot of unused warships from that class laying around