Hitting "Login" from Mantis, going through Deki and logging in, does not return you to your bug.
Related, I think: visiting the Bugs site, when you're already logged in, takes you to the login page with the message, "You're already logged in". This repeats; you're never able to get to the main bug page. Here's a screen shot from Firefox, though I don't imagine it's that helpful: Linking directly to a bug number does work, so it appears to be a problem with the main bug page.