Did this occur during normal playback or after you did something like editing/deleting a track?
I used the plugin before, but disbaled it in the Panels Configuration and I certainly edited/deleted some tracks.
I reproduced the error by starting musicbee enabling the plugin in Configure Layout -> Arrange Panels... Then I resized the panel and double clicked on a album cover. Playback starts. Then I switched to an other tab. When returning to the coverflow tab it was blank and the error message appeared. Playback continues. After restarting musicbee, the slides reappear, but after switching to an other tab, it is gone again and the error message appears again.
I expect I'll probably be dealing with the UPnP stuff for another week and then I'll focus on this.
Thanks! Take your time!