I'm having the same problem, its accepting my username and password, but takes me back to the log in screen. I'm using both firefox and safari (on a mac) and have cleared the cache and cookies, but still won't work.

I'm using the latest version of opn, downloaded today on a php/mysql database with my host - what am i doing wrong?

I'm having this problem. It is addressed in this thread but not resolved.

To summarize:
-New to OPN
-Just installed
-PHP 5.2.9
-MySQL 5.0.77
-Points to a login page at mydomain/html/index.php
-Nothing to do, nowhere to go from there.
-Can't find admin page
-Tried pointing to mydomain/html/admin.php but it redirects to same page above.

TIA for your help.