[Marco van Herwaarden]

To troubleshoot this, please remove any hacks and disable your plugins, then see if you still have this problem.
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.

Please upload tools.php from the 'do_not_upload' folder of your vBulletin distribution to your admincp directory. Now point your browser to admincp/tools.php.

Reset the cookie Domain and Path to the default values.
Once your problem is fixed please remove tools.php again from the server.
If this does not solve your problems, please clear your browser cookies and cache, restart your browser and try again.

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.