You've found the cause of the problem yourself then. " if certain settings on,". Maybe find out which is causing the problems and post them?
Check the URL defined in vBulletin Options and make sure it is set correctly (read the description of the option)
You know the global settings - Forum Url *add forum url without adding http:...."* When I don't add that forum name, everything works but when I do, it doubles up :S
The problem is, if I leave this empty, I get a few faults still. If I fill it in, it doubles up :S