"Create new user" does nothing

Hello.

Client is running a 5.x version of Drupal.

Modules below are activated:

Ad 5.x-1.3-2 An advertising system for Drupal powered websites.
Embed 5.x-1.3-2 Embed ads in content.
Image 5.x-1.3-2 Enhances the ad module to support banner ads.
Notify 5.x-1.3-2 Receive email notifications regarding ads.
Report 5.x-1.3-2 Provides comprehensive charts and reports about advertising statistics.
Text 5.x-1.3-2 Enhances the ad module to support static text ads.

Adsense basic 5.x-2.x-dev Configuration for Adsense for a basic single owner site, with no revenue sharing.

Audio 5.x-1.4 Allows you to upload and playback audio files.
Audio getID3 5.x-1.4 Adds the ability to read artist info from and write to audio files.
Audio Images 5.x-1.4 Adds the ability to attach album art to audio nodes.
Audio Import 5.x-1.4 Allows audio module admins to import batches of audio files.
Audio iTunes 5.x-1.4 An iTunes specific RSS plugin for views feeds.

Aggregator 5.12 Aggregates syndicated content (RSS, RDF, and Atom feeds).
Blog 5.12 Enables keeping easily and regularly updated user web pages or blogs.
Color 5.12 Allows the user to change the color scheme of certain themes.
Comment 5.12 Allows users to comment on and discuss published content.
Contact 5.12 Enables the use of both personal and site-wide contact forms.
Drupal 5.12 Lets you register your site with a central server and improve ranking of Drupal projects by posting information on your installed modules and themes
Forum 5.12 Enables threaded discussions about general topics.
Help 5.12 Manages the display of online help.
Poll 5.12 Allows your site to capture votes on different topics in the form of multiple choice questions.
Profile 5.12 Supports configurable user profiles.
Search 5.12 Enables site-wide keyword searching.
Statistics 5.12 Logs access statistics for your site.
Taxonomy 5.12 Enables the categorization of content.
Poll 5.12 Allows your site to capture votes on different topics in the form of multiple choice questions.
Profile 5.12 Supports configurable user profiles.
Search 5.12 Enables site-wide keyword searching.
Statistics 5.12 Logs access statistics for your site.
Taxonomy 5.12 Enables the categorization of content.

SEO Checklist 5.x-1.3-1 A Search Engine Optimization checklist for Drupal sites.

Image 5.x-1.8 Allows uploading, resizing and viewing of images.

User registration notification 5.x-1.8 Notifies administrator of new user registrations.

Acidfree 5.x-1.x-dev A simple unified media management module for Image and Video modules
AdSense 5.x-1.6 Display Google AdSense ads on your site to earn revenue.
Amazon associate tools 5.x-1.5 Creates associate links to Amazon.com books.
AWeber Integration 5.x-1.0-beta1 Integrate and manage your site users as AWeber leads.
Backup and Migrate 5.x-1.1 Backup or migrate the Drupal Database quickly and without unnecessary data.
Click2bookmark 5.x-1.4 Allow users to bookmark nodes and retrieve them in a block.
Diggthis 5.x-1.3 Display a Digg this button in nodes of enabled types. See http://digg.com/tools/integrate
Global Redirect 5.x-1.1 Searches for an alias of the current URL and 301 redirects if found. Stops duplicate content arising when path module is enabled.
Google Analytics 5.x-1.3 Adds Google Analytics javascript tracking code to all your site's pages.
LoginToboggan 5.x-1.3 Improves Drupal's login system.
Meta tags 5.x-1.8-rc1 Allows users to add meta tags, eg keywords or description.
multiping 5.x-1.3 An enhanced ping module for pinging multiple sites
nodeauthor 5.x-1.3 A simple module that adds an 'About Author' field to the bottom of selected node types which can be populated by users when they edit their profile.
Page Title 5.x-1.1 Enhanced control over the page title (in the tag).
Pathauto 5.x-1.2 Provides a mechanism for modules to automatically generate aliases for the content they manage.
Sections 5.x-1.2 Allows you to define sections of your site and apply themes to those sections.
Service links 5.x-1.0 Add Digg, del.icio.us, reddit, Technorati etc. links to nodes.
Taxonomy breadcrumb 5.x-1.4 Enables taxonomy based breadcrumbs and allows for node assosciations with taxonomy terms.
TinyMCE 5.x-1.9 The most popular WYSIWYG editor for advanced content editing.
Token 5.x-1.11 Provides a shared API for replacement of textual placeholders with actual data.
Vocabulary List Nodes 5.x-1.0 Provides nodes listing by vocabulary with standart pager.

AdSense Injector 5.x-2.5-1 Insert adsense ads into full-page node view, and front page or other list views automatically.

CAPTCHA 5.x-3.2 Base CAPTCHA module for adding challenges to arbitrary forms.
reCAPTCHA 5.x-2.5 Uses the reCAPTCHA web service to improve the CAPTCHA system.

Google Video 5.x-1.x-dev Enable Google video support for Video module.
Upload Video 5.x-1.x-dev Enable Uploaded video support for Video module.
URL Video 5.x-1.x-dev Enable URL video support for Video module.
Video 5.x-1.x-dev Allows video nodes.
Video Image 5.x-1.x-dev Enable thumbnails support for video module.
Youtube Video 5.x-1.x-dev Enable Youtube videos support for Video module.

Views 5.x-1.6 The views module creates customized views of node lists.
Views RSS 5.x-1.6 RSS plugin for the views feed selector argument.
Views Theme Wizard 5.x-1.6 The views theme wizard helps create stub theming for views.
Views UI 5.x-1.6 The Views UI module allows you to create and edit views.

XML Sitemap 5.x-1.4 Creates an XML site map in accordance with the sitemaps.org specification.
XML Sitemap: Engines 5.x-1.4 Submits site map to search engines.
XML Sitemap: Node 5.x-1.4 Adds nodes to the site map.
XML Sitemap: Term 5.x-1.4 Adds taxonomy terms to the site map.
XML Sitemap: User 5.x-1.4 Adds user profiles to the site map.

When admin account goes to DOMAIN/admin/user/user/create and fills out the form and hits submit nothing happens.

Nothing meaning:
- no errors in logs
- no account created
- sends back to DOMAIN/admin/user

I'm not sure where / what this could be caused by.

Any thoughts? o.O