You may try other, simpler-format .htaccess file, just to check if it will work with PHP. For example:
RewriteRule ^test\.htm$ ./test.php
Now if you type in the browser's URL bar 'test.html' and if it points to $_SERVER array output of test.php, it means everything should work. Else... investigate it more deeply.
Also make sure you don't have anything else in .htaccess file which may conflict with miniBB rules.
The error you have, is a server-side error
. Just to repeat - it's not miniBB fault
Besides of it all, we don't have any more recommendations. It appears purely some fault on your side, which is impossible to determine without debugging and seeing files.