Trying to clean the controller didn’t work for me either. It also made the situation worse by damaging the delicate flex cable and making the “set” button non functioning. I also broke one of the small pins at the back of the 4 way button so I had to look for a whole new unit. Fortunately, I could find one from aliexpress. I replaced the part and now everything is ok. The feel on the set button is not very good though. But it is way better than a non functioning button. I bought the whole unit for 26 dollars but if you haven’t damaged your plastic button part you can find the flex cable alone for much cheaper. Here is my order: https://www.aliexpress.com/item/For-Canon-6D-SET-key-to-confirm-6D-key-back-button-on-the-back-wheel-with/32859645101.html?spm=a2g0s.9042311.0.0.60dd4c4dS8iG3f
I really appreciate this topic and I thank the author and every contributer who saved me from paying hundred dollars to Canon service here in my country. Great job dudes!