I would like to know how to prevent the redirect warning from submitexpress. My forum will not get indexed if there is a redirect rule in a php script.

What kind of warning do you mean, what is submitexpress and how miniBB should be related to that?..

the forum url : example.com/forum redirects to example.com/forum/ is this bad for SE's ?
I saw my forum is indexed now but with the wrong description of threads, damn.

I changed the text for the index.php file :

<?php
/*
I think this text was indexed ........
*/
$unset=array

Deleted the text now it is like this :

<?php
/*

*/
$unset=array

Is this ok ?

It doesn't matter for SE's at all. Ideally it should always work that way (and it depends on the configuration of your server). If you enter minibb.com/forums for example, it will be redirected to minibb.com/forums/ - and it is correct.

The text you've changed won't be available on the page at all. And you can't remove it because it's part of the GPL license, i.e. your copyright agreement.

Description of threads should be build dynamically depending on the content of the first post.

Recently I've read the interesting point of view about SEO and in many aspects support it. Study it carefully and you will be wondered that it's not important how the pages are build, but what's on them.