I also have this problem sometimes, but couldn't establish a pattern about when it happens... My workaround to get it working again is simpler than removing and adding the widget, I just tap the music title to launch Media Player, and then the widget buttons get functional again. Due to this behavior I think the problem may have something to do with the widget accessing pulseaudio or other music related daemons/binaries/scripts, but I don't have enough technical background about how multimedia works on Maemo, so it is just a guess...