Question: Why did you ImPex from vBulletin 3 to vBulletin 4? Why not just upgrade instead? Upgrading should be easier than using the ImPex system unless there is an issue I've not encountered before.
Next...try globally disabling your mods to see if a mod maybe causing this issue. Many mods from vB3 do not work in vB4 as the coding is different from one to the other so it could be your problem.
You will need a HTML editor like Dreamweaver or use NotePad.
DO NOT use MS Word or Word Pad. Global disable: open your
includes>config.php file and find:
and under place:
Code:
define('DISABLE_HOOKS', true);
Save and upload again to your server in the includes directory.
If you find that it works after disabling the plugins globally then re-enable the plugins by putting // in front of the code so it looks like this:
Code:
//define('DISABLE_HOOKS', true);
Save and upload again.
Then you will have to go through your mods one by one until you find the one that is causing the conflict.