I've seen people complaining about not getting the seo mod to work and seeing white pages etc...
Here is a solution that people often miss. Open up your .htaccess file and edit the part in blue to reflect the path to your forum as you would see it in an ftp program:
Lets say you have a site and your forum is this: http://mysite.host.com/forum then you edit the part in blue to say "RewriteBase /forum"
Or if it's: http://mysite.host.com/mybb change it to "RewriteBase /mybb" & so on and so forth.
# //seo_mod_start
RewriteEngine On
# Uncomment the following and add your forum path if rewrites arent working properly
RewriteBase /
RewriteRule ^index.html$ index.php [L,NE]
RewriteRule ^(.*)-t-([0-9]+).html(.*)$ showthread.php?tid=$2$3 [QSA,L]
RewriteRule ^(.*)-t-([0-9]+)-([0-9]+).html$ showthread.php?tid=$2&page=$3 [QSA,L]
RewriteRule ^(.*)-f-([0-9]+).html(.*)$ forumdisplay.php?fid=$2$3 [QSA,L]
RewriteRule ^(.*)-u-([0-9]+).html$ member.php?action=profile&uid=$2 [QSA,L]
RewriteRule ^members/(.*)-([0-9]+).html$ member.php?action=profile&uid=$2 [QSA,L]
RewriteRule ^(.*)/(.*)-f-([0-9]+).html(.*)$ forumdisplay.php?fid=$2$3 [QSA,L]
RewriteRule ^(.*)-f-([0-9]+)-([0-9]+).html(.*)$ forumdisplay.php?fid=$2&page=$3 [QSA,L]
RewriteRule ^(.*)-f-([0-9]+)-([a-z]+)(-|-[a-z]+)-([0-9]+)-([0-9]+).html(.*)$ forumdisplay.php?fid=$2&sortby=$3&order=$4&datecut=$5&page=$6$7 [L]
RewriteRule ^(.*)-a-([0-9]+).html$ announcements.php?aid=$2 [L]
# //seo_mod_end