thank you so much, dear holuschi! had the same problem (see my post).
the reason was: i did set a filter on a certain folder (show ONLY (!) folder ABC). later i deleted the folder ABC.
musicbee couldn't find the folder ABC anymore AND the filtersetting gave permission to show ONLY folder ABC.
steven, is there a workaround for this problem? perhaps: "let the filtersettings change ALLTHETIME?"