If Action! is open when opening Paint.net, or if you have Paint.net open and then open Action! Paint.net crashes and says that Action! may be the problem (as it seems to be).
Also wondering why the updater for 4.10.2.0 fails to update (I was on an older version but I cannot remember which, I think it might have been 4.8-4.9, but I don't know.) when it never did this before.