Registration problem > can't display next page

[Tigatoday]

Hello there,

I have a weird problem. Some of the new users who want to register get a "Internet Explorer cannot display the webpage" page after submitting their registration form. Some, not all. I myself get it too when attempting to register with a new username.

Version:
vBulletin® Version 3.6.8

Add ons:
vBulletin 3.6.8
Benutzer, die das Forum heute besucht haben 4.17 Anzeige der Benutzer, die das Forum heute schon besucht haben.
Cyb - Advanced Forum Statistics 5.5 Cyb - Advanced Forum Statistics
Cyb - Advanced Permissions Based on Post Count 4.0 Cyb - Advanced Permissions Based on Post Count Wijzigen Version überprüfen Deaktivieren Exporteer Deinstallieren
Cyb - PN System Verbesserungen 1.4 Cyb - PN System Verbesserungen
Cyb - Top Posters In Month X 1.5 Cyb - Top Posters In Month X
Geburtstags-Glückwunsch-Thema 1.5.0
Multiple Login Detector 1.03 Cookie-based multiple account login detector
Restrict URLS 0.1 Restricts users from posting URLs until they have an admin defined number of posts.
Separate Sticky and Normal Threads 1.0.5 Separate Sticky and Normal Threads
Validate E-mail 1.0 Checks that the user supplied e-mail address is a real address.
Weekly Stats 1.7.0 Displays and Emails weekly stats for current forum
Yet Another Mass Private Message System 1.0.1 Allow Admin to send mass Private Message to users

Anybody any clue?
Thx!