I'm sure it's some kind of incompatibility with any of the following plugins my client installed:
Akismet
Deactivate | Edit | Settings
Used by millions, Akismet is quite possibly the best way in the world to protect your blog from comment and trackback spam. It keeps your site protected from spam even while you sleep. To get started: 1) Click the "Activate" link to the left of this description, 2) Sign up for an Akismet API key, and 3) Go to your Akismet configuration page, and save your API key.
Version 2.5.3 | By Automattic | Visit plugin site
Select All in One SEO Pack All in One SEO Pack
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Out-of-the-box SEO for your WordPress blog. Options configuration panel | Upgrade to Pro Version | Donate | Support | Amazon Wishlist
Version 1.6.13.4 | By Michael Torbert | Visit plugin site
Select Broken Link Checker Broken Link Checker
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Checks your blog for broken links and missing images and notifies you on the dashboard if any are found.
Version 1.3.1 | By Janis Elsts | Visit plugin site
Select cforms cforms
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cformsII offers unparalleled flexibility in deploying contact forms across your blog. Features include: comprehensive SPAM protection, Ajax support, Backup & Restore, Multi-Recipients, Role Manager support, Database tracking and many more. Please see ____HISTORY.txt for what's new and current bugfixes.
Version 12.2 | By Oliver Seidel | Visit plugin site
Select Custom Author Permalink Custom Author Permalink
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This plugin is deprecated (although still working) and will no longer be maintained. Its functionality has been re-written and improved on WP htaccess Control which you should be using instead.
Version 1.2 | By dardna | Visit plugin site
Select Customize Your Community Customize Your Community
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Uses your current theme to display login, registration and profile pages instead of the WordPress look and feel.
Version 1.2.1 | By Joost de Valk | Visit plugin site
Select Dean's FCKEditor For WordPress Dean's FCKEditor For WordPress
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Replaces the default WordPress editor with CKeditor
Version 3.3.1 | By Dean Lee | Visit plugin site
Select Google XML Sitemaps Google XML Sitemaps
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This plugin will generate a special XML sitemap which will help search engines like Google, Yahoo, Bing and Ask.com to better index your blog.
Version 3.2.6 | By Arne Brachhold | Visit plugin site | Settings | FAQ | Support | Donate
Select ILC Thickbox ILC Thickbox
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Displays images creating a thumbnail and adding WordPress built-in Thickbox.
Version 1.0 | By Elliot | Visit plugin site
Select Jetpack by WordPress.com Jetpack by WordPress.com
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Bring the power of the WordPress.com cloud to your self-hosted WordPress. Jetpack enables you to connect your blog to a WordPress.com account to use the powerful features normally only available to WordPress.com users.
Version 1.1.3 | By Automattic | Visit plugin site
Select Kimili Flash Embed Kimili Flash Embed
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Provides a full WordPress interface for SWFObject - the best way to embed Flash on your site.
Version 2.2 | By Michael Bester | Visit plugin site
Select MicroPoll MicroPoll
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Easily integrate web-based polls from MicroPoll into WordPress
Version 1.0 | By Chris Abernethy | Visit plugin site
Select More Fields More Fields
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Add more input boxes to use on the write/edit page.
Version 2.1 | By Henrik Melin, Kal Ström | Visit plugin site | Settings | Support | Donate
Select Post Templates by Category Post Templates by Category
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Use custom single post templates for specified categories.
Version R1.3 | By Kaf Oseo | Visit plugin site
Select PS Auto Sitemap PS Auto Sitemap
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Auto generator of a customizable and designed sitemap page.
Version 1.1.3 | By Hitoshi Omagari | Visit plugin site
Select Quizzin Quizzin
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Quizzin lets you add quizzes to your blog. This plugin is designed to be as easy to use as possible. Quizzes, questions and answers can be added from the admin side. This will appear in your post if you add a small HTML snippet in your post.
Version 1.01.4 | By Binny V A | Visit plugin site
Select Receive Links Plugin Receive Links Plugin
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A plugin to include Receive Links Client on a WordPress widget
Version 2.4.4 | By Receive Links Support Staff | Visit plugin site
Select Register Plus Register Plus
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WordPress 2.5+ ONLY. Enhance your Registration Page. Add Custom Logo, Password Field, Invitation Codes, Disclaimer, Captcha Validation, Email Validation, User Moderation, Profile Fields and more.
Version 3.5.1 | By Skullbit | Visit plugin site
Select Show authors without posts Show authors without posts
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Shows the authors page even if the author has no post.
Version 1.0.1 | By Simon Brüchner | Visit plugin site
Select ThickBox ThickBox
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Embed ThickBox into your posts and pages.
Version 1.5 | By Christian Schenk | Visit plugin site
Select User Photo User Photo
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Allows users to associate photos with their accounts by accessing their "Your Profile" page. Uploaded images are resized to fit the dimensions specified on the options page; a thumbnail image is also generated. New template tags introduced are: userphoto_the_author_photo, userphoto_the_author_thumbnail, userphoto_comment_author_photo, and userphoto_comment_author_thumbnail. Uploaded images may be moderated by administrators.
Version 0.9.5.1 | By Weston Ruter | Visit plugin site
Select Viper's Video Quicktags Viper's Video Quicktags
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Easily embed videos from various video websites such as YouTube, DailyMotion, and Vimeo into your posts.
Version 6.3.1 | By Viper007Bond | Visit plugin site
Select WordPress.com Stats WordPress.com Stats
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Future upgrades to WordPress.com Stats will only be available in Jetpack.
Version 1.8.5 | By Automattic | Visit plugin site | Enable Jetpack Now!
Select WP-DBManager WP-DBManager
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Manages your WordPress database. Allows you to optimize database, repair database, backup database, restore database, delete backup database , drop/empty tables and run selected queries. Supports automatic scheduling of backing up, optimizing and repairing of database.
Version 2.63 | By Lester 'GaMerZ' Chan | Visit plugin site