Man, that URL looks like it went down a serious rabbit hole! Glad that autorun link finally saved the day and got you back in business.
For future reference, those infinite redirect loops usually happen when the server gets confused by old, cached session data. Wait, actually, I think it's because the server is trying to read your code backwards from right to left, or, more likely, it's just those ancient 1.4 plugin files conflicting with the new core.
Definitely delete that old /plugins/site_upgrade/ folder so your site doesn't try to inception itself again!