For this to work, you need to untick the following option:
Configuration.exe -> Music -> Playlist Settings Tab -> Load Default Playlist on MediaPortal Startup
What was happening is the following:
1. The "Save Current Playlist as default when leaving Mediaportal" saved your current playlist as default.m3u when exiting MP the last time
2. During Startup OneButtonMusic was creating your playlist and starting playback
3. But after that in the Startup sequence the default.m3u gets loaded and overwrites the just created playlist.
Unticking the oprion "Load default ..." doesn't do that anymore and your AutoStart playlist plays fine
Thank you very much, that fixed it. Appreciate you taking the time for the explanation too so I can understand what has happening.