Sidebar Login Version 2.2.4. Noticing other folks here with similar problems but I'm not sure that it's the same.
I have the login form on my homepage using the template tag. After I submit the form with my username and password, I need to refresh or move to a different page for it to recognize that I'm now logged in. It isn't shy about displaying errors if fields are blank or incorrect, the only problem arises when the submission is valid.
The form works just fine in Safari. Testing in IE 7 shortly.
Edit: Just wanted to note - I'm using my admin account only. Haven't tested subscriber or contributor.