I have been running miniBB on my DH-hosted site for almost a year (currently running v1.7f) without problems. Like chell said, I cannot recall having to modify bb_admin.php other than renaming it. In my setup_options.php, I have kept the default settings for cookies:
The miniBB developers–known as ‘Team’ to miniBB support forum users–are very helpful, so I am sure they will be able to help resolve the problem.
All the best,