MIra a ver si el Plugin de SEO lo activaste bien (debes darle a "aplicar cambios" desde el ACP) y como dicen los compañeros revisa el .htaccess
Bueno comento que desactive el google seo y sigue igual aqui les dejo el htacces:
Código PHP:
Options -MultiViews +FollowSymlinks -Indexes
#
# If mod_security is enabled, attempt to disable it.
# - Note, this will work on the majority of hosts but on
# MediaTemple, it is known to cause random Internal Server
# errors. For MediaTemple, please remove the block below
#
<IfModule mod_security.c>
# Turn off mod_security filtering.
SecFilterEngine Off
# The below probably isn't needed, but better safe than sorry.
SecFilterScanPOST Off
</IfModule>
#
# MyBB "search engine friendly" URL rewrites
# - Note, for these to work with MyBB please make sure you have
# the setting enabled in the Admin CP and you have this file
# named .htaccess
#
<IfModule mod_rewrite.c>
RewriteEngine on
# Some hosts require RewriteBase to make RewriteRules work.
RewriteBase /
# Google SEO workaround for search.php highlights:
# Make this rule the first rewrite rule in your .htaccess!
RewriteRule ^([^&]*)&(.*)$ http://www.descargatex.com/$1?$2 [L,QSA,R=301]
# Google SEO 404:
ErrorDocument 404 /misc.php?google_seo_error=404
# Google SEO Sitemap:
RewriteRule ^sitemap\-([^./]+)\.xml$ misc.php?google_seo_sitemap=$1 [L,QSA,NC]
# Google SEO URL Forums:
RewriteRule ^foro\-([^./]+)$ forumdisplay.php?google_seo_forum=$1 [L,QSA,NC]
# Google SEO URL Threads:
RewriteRule ^post\-([^./]+)$ showthread.php?google_seo_thread=$1 [L,QSA,NC]
# Google SEO URL Announcements:
RewriteRule ^anuncio\-([^./]+)$ announcements.php?google_seo_announcement=$1 [L,QSA,NC]
# Google SEO URL Users:
RewriteRule ^miembro\-([^./]+)$ member.php?action=profile&google_seo_user=$1 [L,QSA,NC]
# Google SEO URL Calendars:
RewriteRule ^calendario\-([^./]+)$ calendar.php?google_seo_calendar=$1 [L,QSA,NC]
# Google SEO URL Events:
RewriteRule ^evento\-([^./]+)$ calendar.php?action=event&google_seo_event=$1 [L,QSA,NC]
RewriteRule ^foro-([0-9]+)\.html$ forumdisplay.php?fid=$1 [L,QSA]
RewriteRule ^foro-([0-9]+)-page-([0-9]+)\.html$ forumdisplay.php?fid=$1&page=$2 [L,QSA]
RewriteRule ^post-([0-9]+)\.html$ showthread.php?tid=$1 [L,QSA]
RewriteRule ^post-([0-9]+)-page-([0-9]+)\.html$ showthread.php?tid=$1&page=$2 [L,QSA]
RewriteRule ^post-([0-9]+)-lastpost\.html$ showthread.php?tid=$1&action=lastpost [L,QSA]
RewriteRule ^post-([0-9]+)-nextnewest\.html$ showthread.php?tid=$1&action=nextnewest [L,QSA]
RewriteRule ^post-([0-9]+)-nextoldest\.html$ showthread.php?tid=$1&action=nextoldest [L,QSA]
RewriteRule ^post-([0-9]+)-newpost\.html$ showthread.php?tid=$1&action=newpost [L,QSA]
RewriteRule ^post-([0-9]+)-post-([0-9]+)\.html$ showthread.php?tid=$1&pid=$2 [L,QSA]
RewriteRule ^post-([0-9]+)\.html$ showthread.php?pid=$1 [L,QSA]
RewriteRule ^anuncio-([0-9]+)\.html$ announcements.php?aid=$1 [L,QSA]
RewriteRule ^miembro-([0-9]+)\.html$ member.php?action=profile&uid=$1 [L,QSA]
RewriteRule ^calendario-([0-9]+)\.html$ calendar.php?calendar=$1 [L,QSA]
RewriteRule ^calendario-([0-9]+)-year-([0-9]+)\.html$ calendar.php?action=yearview&calendar=$1&year=$2 [L,QSA]
RewriteRule ^calendario-([0-9]+)-year-([0-9]+)-month-([0-9]+)\.html$ calendar.php?calendar=$1&year=$2&month=$3 [L,QSA]
RewriteRule ^calendario-([0-9]+)-year-([0-9]+)-month-([0-9]+)-day-([0-9]+)\.html$ calendar.php?action=dayview&calendar=$1&year=$2&month=$3&day=$4 [L,QSA]
RewriteRule ^calendario-([0-9]+)-week-(n?[0-9]+)\.html$ calendar.php?action=weekview&calendar=$1&week=$2 [L,QSA]
RewriteRule ^evento-([0-9]+)\.html$ calendar.php?action=event&eid=$1 [L,QSA]
<IfModule mod_env.c>
SetEnv SEO_SUPPORT 1
</IfModule>
</IfModule>
#
# If Apache is compiled with built in mod_deflade/GZIP support
# then GZIP Javascript, CSS, HTML and XML so they're sent to
# the client faster.
#
<IfModule mod_deflate.c>
AddOutputFilterByType DEFLATE application/javascript text/css text/html text/xml
</IfModule>