Just noticed when the mod " Better ship icons on compact carousel " is activated it's not possible anymore to assign a captain to a ship from an other ship.
When trying e.g to assign a captain from standard ship to premium ship you get message that there is no free place in reserve available, even if the reserve has enough free space. Action is to assign caption to ship and not send to reserve, so the error message makes no sense here.
After deactivating the mod assigning captains is possible again.