if i'm picturing this correctly (and i hope i am), your right knee is basically buckling in laterally when you strap in and ride, even when you have a very narrow stance? this could be a flexibility problem. quick check to find out:
set up in a squat stance with your toes pointing out (45 degrees is fine). your heels should be lined up with your shoulders, or a little less. keeping your back and neck straight, squat down until you're bottomed out. i'd say ass to grass but that's a different flexibility issue so get down as low as you can while keeping both feet flat
. you can repeat it with your snowboard stance.
1. at that position, are your thighs at least parallel to the floor? if not, that's a huge
flexibility issue right there that could be related. i'd focus on fixing that; it'll definitely help your long-term health outside of snowboarding. just do the above 5-10 times daily, keep pushing your bottom while keeping your feet flat, and you'll be okay.
2. if you don't have the above problem, the next step is to shove your knees out. like, OUT. pretend you're squeezing the floor between your feet together. if necessary, use your elbows to help. your knees should be able to track over your feet without causing much, if any, discomfort in your knee. if you can't do this or you're experiencing a lot of pain holding this then this could be what's causing your discomfort. that's another stretch to practice.
since you mention it's been years since you got on a snowboard, this came to my mind when other people's solutions didn't help. your flexibility may have flat-out changed. you'll be surprised how much it really helps and it's something many people ignore.
let me know if it works and where you live. i'll fly in for the beer you'll owe me