The dealer has tools to get the regen on.. since it would never work for you on the highway, and it sounds like it's not working for them it does sound like a new DPF is in order. I'm assuming your still under warranty?
If it's a great deal, and doesn't hurt you wallet I would do it just for the warranty. But that's just me!
But, I do see your point about not having any issues. It seems hit and miss..
Welcome Paul, a few threads here with that same code:
https://www.ecodieselram.com/forum/threads/my-first-error-code-less-than-a-week-of-ownership.1612/
It seems it's one of those ghost codes that appear in really cold climates. Does that apply to you?
15K is a little to long for my liking, but if it's working for you I can't complain about that. I would be interested to see an oil analysis on your oil after 15K.