Hello NAP -
Just in case you missed it, Jasper Warwick's ("This probably wont work but still worth a try") solution near bottom of this thread seems to have worked for me. I had the exact same Playlist behavior as you described above for the past year, with no fix. I just learned to live with it. As Jasper says, "it's worth a try".
Good luck.
Mitch
The steps in some detail just for clarity's sake (not knowing your comfort level with these matters)...
1. Rename your problem playlist to "Invalid" (w/cap). Select your problem playlist, then use 3-dot dropdown toolbar next to that Playlist name on Web UI. (Rename this playlist).
2. Delete it (again use 3-dot dropdown toolbar next to that Playlist name on Web UI. (Delete this playlist).
3. Run a Scan Disk from Web UI using "Settings and Status" toolbox (sprocketed gear at top of Web UI). Wait for Scan to complete.
4. Do a Save from the B2 itself by holding down the Play/Stop button on B2. When "Saving" appears, release Play/Stop button.
5. When clock reappears on B2 display, with no other commands in progress and no music playing, power down the unit (however you do this... unplug from wall or if you have a power-switch, use that).
6. Walk away for a bit, maybe have a bite to eat, then return and power up again.
7. Hopefully your problem playlist will be forever gone to wherever it is these things go.
As I said, this worked just now on my deck after a year or so of fighting, then surrendering. If it doesn't do the trick, near bottom of this thread, Paul from Brennan has provided instructions for sending him a "debug" log.
Good luck.
Mitch