I think the salt changed for 4.0.1, so you will need to update your templates.
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
Set it to default and have users login using this style - do you still have the same problem?
If you have further problems, please post over on vbulletin.com. See this sticky -
Installation Issues / Questions or bug reports regarding default vBulletin 4.0