It's an oldie (Jan 2021) but might still be relevant. I was playing in my friend's game and roll20 seemed to be having a massive problem with processing the stuff on screen, creating all sorts of rapidly changing visual glitches, with the image below blacking out most of the screen except this weird semi-transparent grid. It changed whenever I moved the camera, and seemed to have something to do with the purple portal in the middle being drawn in. It didn't affect anyone else.