Noticed that folder too, after Craglong went life I think. Seems to be recreated by the game if missing.
It might be a place to store addons/saved vars that are shared beween the clients, but I have not tested it yet (since there was nothing in the patchnotes about it it is propably not ready).
No idea about the other half of your question. Wich addosn are you talking about? It is not uncommom the reset userdata when a major version switch occured and the saved var accessor object provides functions to automatially track if the exisitng data is invalid.
Also note that there is quite a delay between reloadui and the data being written to disk.