What an absolute legend! I've been plagued with this issue for months, getting considerably worse over the last two weeks. Not wanting to RMA, I gave this a go on my 5 month old M65 Elite, but without the sandpaper.
VOILA! Mouse back to working order again :) thank you so much for posting this! If it happens again, I'll be buying a new mouse, and steadily changing out my peripherals for another brand, as over the years I have seen Corsair are charging more for their products whilst using cheaper parts. This being the best example.
For now, my mouse is working, and I'm happy again :)