what changes you made in /wp-includes/default-constants.php ? all, some, one or none?
If you mean if I've edited this file during the installation, then the answer is: I did not modify this file at all. I've installed WordPress just following their standard installation guide, i.e. put the database information using the form fields during installation, and put the initial admin info via the similar form. I didn't went into modifying their file. It was just a pure clean installation from scratch.
May be you could try the same. In some sub-folder, re-install Wordpress and install the newer version of miniBB, following standard README guides. If it works, then you could switch to the original database. At least, you must make sure the standard installations work
on your server, without trying to upgrade a customized version.