That could mean you have set up miniBB in the improper encoding. First of all, miniBB and your website should have the same encoding set. Second, miniBB and your database should have the same encoding. There are usually many problems coming with specific symbols, so we recommend always use UTF-8 as the default encoding for your database, miniBB and website. Here you can read how to
my message in Upgrading thread may help you to get a clue about the database. Unfortunately, encoding issue is always custom so I can't give more information on this subject...