I have run in to this same problem. Using mysqli, I get an error about a bad password, whether it is wrong or not.
Using mysql, a bad login shows an error, but a good login clears out the login form and then does nothing.
None of my folder names contain special characters.
I am running on IIS 7.5 FastCGI with PHP 5.3.2 and the latest hMailServer.
hMailServer logs show a successful login followed by a logout after fetching various parameters about the inbox.
The IMAP connection uses SSL
Any ideas? Should I be using the SVN?