Ok, I tried installing an empty blog and upgrading it to WordPress 3.7. Same exact problem.
This is a dedicated server. I guess I should try creating an empty blog and upgrading that (as suggested in the other thread).
Ipstenu, it was this theme:
http://www.theme-time.com/17/
Krishna, thanks for the advice.
Thanks. The problem goes away when switching to the default theme. I guess the “Rainbow Style” theme is incompatible with 3.7. Being redirected to an error page made me think that perhaps there was more wrong.
After re-upgrading, I was still redirected to an error page, but I tried reloading the page (/about.php?updated) and it loads just fine (odd).
Thanks again! 🙂
