Firstly check the M10 bolt the rudder pivots on hasn’t been over tightened by mistake. Very occasionally after first use or at the start of the season (if you have revarnished at the end of the previous season) the servo pendulum rudder “takes up” (absorbs a small amount of water) and can become tight in the sideplates. This is a natural occurrence in wood and in boat building is what seals the joints in wooden boats. It can be easily remedied during the end of season maintenance.
It is very hard to judge exactly how much each rudder will “take up” or how thick each applied coat of varnish is. It may mean you will need to remove a small amount of material from where the servo rudder sits in the side plates using a random orbital sander and re-varnish as instructed in the maintenance guide.