I think it has something to do with Mass Ownership Tweaks...I get the same red text, as if I'm trespassing, or stealing when I try to go into Doc Mitchell's house in Goodsprings. I've also seen a couple posts on that mod page about the same issue with Doc Mitchell's. I tried deleting the Goodspring and Helios One lines in the .ini, like is mentioned on the page, but that didn't do anything. The last version of the guide I had the same issue with Doc Mitchell's, and I turned off that mod, and the problem was resolved with Doc Mitchell's, but with this version of the guide, I can't turn that mod off because the STEP patch requires it as a master.
I'm not sure if that is the actual problem, but I never had any troubles before with that quest on any past modded or vanilla playthroughs.