My 'Now Playing' playlist had 101 entries--all of the same song. When I tried to 'remove' an entry it did not go away. When I tried to play a different song from my library, that duplicated song played.
Prior to running MusicBee I moved to a new computer and moved the music from the C: drive to the D: drive.
I deleted the MusicBee\NowPlaying.mbp file and the problem persisted.
The error log did not contain any entries.
Solution: I opened the .MBP files in an editor and replaced C:\Music with D:\Music.
I am running version 3.1.6501.