This has happened for over a year now. Whenever I pick Smash and choose my ruleset, it takes me to Stage Select but it doesn't let me pick the stage, it just shows the stage select for a second and takes me back to Ruleset Select. I've heard that it's because of a UMM and you need to delete it so I deleted the folder and reinstalled Super Smash Bros Ultimate. But it still has the same issue. It'd be pretty appreciated if anyone can help! 
