Have you tried simply backing off the tension on the nut & bolt that your rudder pivots on?

Disassemble your rudder, look for signs of rubbing or other damage that might have been caused by something getting stuck that you didn't put there. You may be able to sand away enough epoxy to gain back some clearance but if it was tight at the first use it's probably too tight to avoid this happening again.

If you can sand enough off to give it some clearance when re-assembled, try using a paste wax on all four surfaces before you put it back together. There's also powdered graphite you could add to another coat of epoxy on the rudder's faces inside the cassette to provide a more durable lubrication for those moving parts.

You have a router?

Were it me in your position I'd use a router to take off maybe a millimeter off both sides of the rudder where it's hidden (both in use as well as kicked up) under the cassette's sides. Then epoxy as before, with a mind to keeping it uniform & no drips. Maybe get some of that graphite powder to add to the epoxy too.

Or make a new rudder... or give CLC a call tomorrow, see of they'll sell you a new rudder blank you then can finish with a mind to ensuring it ends up thinner than your original.

As it is you don't know what's causing the binding 'cause you can't see in there. Kick-ups can and will jam from a bit of debris that gets caught at an edge, why it's a Good Idea to build 'em a little loose. They can wobble a bit on their pivot bolt & still do a good job of steering. 

Unless you'd be sanding down into layers/materials you don't want sanded away, sanding is certainly an option, whether inside the rudder head cheeks or on the upper part of the blade.  I'd pay special attendtino to really making sure the drips you sanded out inside the cheeks aren't still high points. Some other thoughts:

This is an instance where knocking the gloss off of the finishes will cause them to move more easily.  Maybe just rubbing all surfaces with a brown scotchbrite pad.

Simply working the rudder up and down a number of times might break it in sufficiently to get it moving freely.  The rudder will always loosen up with use.

Make sure your pivot bolt isn't tight, squeezing the cheeks together.

Use soap, parafin wax or silicone "butter" grease on assembly.  The grease will last longest - it has the slight possibility of picking up sand or grit so consider that if you'll be laying your rudder down on a beach or something.

Your rudder is already too tight, so this probably doesn't apply in this case, but I've put rudders together with "giant" (4-6" diameter) plastic washers on each side made out of cottage cheese container tops or pieces cut out of those thin, roll-up plastic sleds.  These washers kept the rudder pivot moving freely and let me tighted down on the pivot bolt a little bit.

Really, other than making sure you don't create a bare-wood situation, there is no right or wrong answer and it all depends on how much it will take to get the rudder moving freely in your situation.  Just pick a couple of options that seem best for your particular situation.

Thanks for all the info and tips. 

The bolt is not even tightned yet, i just ran the bolt through the hole to hold it. So it is not that. It was already hard to pull in the blade in the cassete in the first place. I had already sanded the blade by a lot, if I send a mm more I will strip the veneer completely,  I'm cautios of sending more and having a weak blade that can brake with impact or while taking.

I will check for any more solid drips that might need sanding but I think the inner piece of the cassete should be a mm or 2 thicker. Unfortunatelly I'd dealt some wonky measures from CLC in my built (among a few minor things I had wrong mast size which was replaced by them promptly and wrong mast thwart size too which I didn't replaced because when I notice was already too eager to sail). 

I have yet to finish assemble my KUR, but my initial trial installation, it was really tight.  I marked the hidden section of the rudder, and then sanded both sides, not sure how much, but I sanded both sides, amost through to the next layer of plywood, but did not break through.  After that, it seem to move ok.  I epoxied it, sanded it, and still seems ok.

One idea I did not do, but thought of doing was to spread the outside parts of it gently and slightly.   

Guedelha from what you're describing I'd get on the phone to CLC tomorrow & explain what you're seeing.

If the inner spacers of the rudder casette aren't greater in thickness than your rudder blank it's inevitable that your rudder will bind. One needs thicker spacers, or needs to add something in addition to the spacers when assembling the casette so there's ample clearance for the rudder to move freely.

If such a condition had been noted before this, adding a few layers of 4 oz. fiberglass cloth – bedded in epoxy – would have provided the thickness needed to ensure adequate clearance. 

I've built all of one CLC kit, a Waterlust expedition canoe. I was careful during the build process to take nothing for granted, including that the rudder's casette would allow the rudder, once installed, to move freely. It's easy enough to measure the thickness of kit components before they're assembled; it's not so easy to modify assemblies when you discover later on that there's a problem.

And it's not always mentioned in the build manuals what a customer needs to be watchful for or the manuals would have to be so comprehensive (read as expensive to produce) that they'd cost more to provide than the kit parts themselves.

Call CLC tomorrow or e-mail them. They want your project to be a success, they'll help you in whatever way they can to get you over this hiccup.

   I second the idea of fiberglass. I inherited a Skerry that was in pretty sad condition and restored it. The rudder, which like the boat was already built did not operate well as the blade friction was such that the attached line would not move the blade: useless!

I disassembled the blade and sanded both sides of the head of the blade (part pivoting between the cheeks). I sanded through the first ply layer on BOTH sides. I, then applied a layer of leftover fiberglass from another boat to the entire blade. I filled the weave, sanded and RE-varnished the whole blade before reinstalling on the pivot. Now, the rudder moves freely and despite the thinning of the ply, the fiberglass maintains the strength needed. 
 

I have used the boat, numerous times and the rudder holds strong and moves freely. I would venture to say that a similar remedy would probably solve your stuck rudder issues, well.

Good luck 😊

Here is a linked folder with two pictures of the rudder blade where you can see the amount of thickness removed based on ply lines:   https://www.dropbox.com/sh/zlq0yl95ogh8h53/AADt45aTHh1chd7VSk4ya5iPa?dl=0