Kafkaesqui! You just solved MY problem, too! I was having the same issue as Soprano and thankfully, found this thread. I turned off a plug-in I had that prevented my site from being loaded into a frame, and it fixed the problem! Thanks so much! 🙂
I absolutely backed up ALL my files, as well as my database. 🙂
Unfortunately, I didn’t see the info you posted about upgrading from 1.2 (though I’m pretty sure I was already at 1.5). If that’s so, which I believe it is, then what could be the issue?