I get this every so often -- SMF and I go round and round with the logins. Not today, but it happened to me yesterday. I just happened to stumble on a trick that you might want to try --
If you normally click on "New posts" like I do, try clicking instead on the "Forum" button below the big SMF banner -- or any hard-coded link that's right below the banner. The point is to click on any link that's not related to some type of search (in other words, don't click on "new posts", "today's posts", "my posts", etc).
This tactic took me to a "normal" screen where it shows I'm logged in. SMF seemed to work okay from there.