Unfortunately the best way is to just put the old folder back and start adding underscores to folder names to hide them from LB and see where the issue appears. Most of the time I've heard this it's because of older malformed images tied to MAME games gotten from EmuMovies, I believe they've since been removed but if you haven't updated your media in a while it could potentially be the case. Does the error occur after selecting a specific game? With where this stack trace is showing I'd have assume it occurred when selecting a game, I'd just start with that game and remove the images to see if it fixes it, then potentially rescrape media on the platform that game is in.