Try this:
Create a new style with no parent:
- Styles & Templates > Style Manager > Add New Style
- Parent Style: No Parent Style
- Title: Default vBulletin
- Allow User Selection: Yes
- Save
Then browse the site using that totally default vbulletin style - do you still have the same problem?
If your issue is gone with the default thread, then that means it is a problem with your style. Read this to fix it -
Implementing CSRF Protection in modifications