It seems any kind of exception condition drops me in the main site page. for instance:

Examples:

If I link to an avatar stored on my own domain rather than uploading it to you, regardless if its size and 'weight' agree with your limits, away I go to the main page. No error messages, no nothing - just yoink! Plop! Out of the forums and sitting on the main page scratching my head.

Attempting to post a reply to a locked thread. ---> Dumped back to main page.

While nipping things that can cause issues in the bud by turfing people out of the forums might be expedient, it does nothing but aggravate users if they are not being told why that happened.

Please look into this.

Note: I have built several sites based on VBulletin - most of them my own but also for others. VBulletin while a very secure and robust package, does require its cookies to be located in the right places. It also requires you to properly manage error conditions. It will take you some time to learn this, but once you have it all down you will have a really nice discussion board here.