@hifi : That's a pretty funny reply. If I knew how the error was caused, I wouldn't be asking for help. And if you want to see messages on the other board, and it requires registration, well then I guess you'll have to register
Anyone else have something constructive to contribute?
I didn't ask what is causing the error (I can read that it's combination of that one theme and this plugin), but how it's showing up. Error message, DB error, and stuff like that. At my point of view (by reading your message) it seems to be problem with template, because I have couple of pages with different templates, and registration is working just fine.
And actually (when remembered problems that had with some components): What mode the sh404SEF was in? index.php?, index.php or .htaccess?