[Marco van Herwaarden]

I don't se any obvious reaon in your PHP config.

To troubleshoot this, please remove any hacks and disable your plugins, then see if you still have this problem (this includes disabling vBSEO).
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php

define('DISABLE_HOOKS', true);

If so, reupload all the original vB non-image files (except install.php). Make sure you upload these in ASCII format and overwrite the ones on the server.

If your problem still exists, please create a new Style with no Parent set. Now browse the forum using this new style, do you still have the same problems? If you are troubleshooting login or guest issues, you will need to set this style as board default in the vBulletin Options.

If all of this fails, please open a support ticket.