I tried deactivating all plugins apart from MaxButtons, but the problem is still the same. Here is the System Info as copied from MaxButtons.
—– Begin System Info —–
WordPress Version: 3.8.1
PHP Version: 5.3.27
MySQL Version: 5.1.56-log
Web Server: Apache
WordPress URL: http://www.xxx.com
Home URL: http://www.xxx.com
PHP cURL Support: Yes
PHP GD Support: Yes
PHP Memory Limit: 256M
PHP Post Max Size: 7M
PHP Upload Max Size: 7M
WP_DEBUG: Disabled
Multi-Site Active: No
Operating System: Mac
Browser: Google Chrome 33.0.1750.152
User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/33.0.1750.152 Safari/537.36
Active Theme:
– DM Twenty Twelve Child theme 1.0
http://www.jazz-piano.co.uk/wordpress/wp-content/themes/twentytwelve-DM-childtheme
Active Plugins:
– Akismet 2.6.0
http://akismet.com/?return=true
– Broken Link Checker 1.9.3
http://w-shadow.com/blog/2007/08/05/broken-link-checker-for-wordpress/
– Custom Menu 1.8
http://www.evolonix.com/wordpress/plugins/custom-menu
– Custom Menu Shortcode 1.0
http://www.basvanderlans.nl/wordpress-plugin/custom-menu-shortcode
– Display Widgets 2.03
http://strategy11.com/display-widgets/
– Duplicate Post 2.6
http://lopo.it/duplicate-post-plugin/
– Fonts 1.0
http://wpsites.net/plugins/fonts/
– Google Analytics for WordPress 4.3.5
http://yoast.com/wordpress/google-analytics/#utm_source=wordpress&utm_medium=plugin&utm_campaign=wpgaplugin&utm_content=v420
– IK Facebook Plugin 2.6.3.4
http://goldplugins.com/documentation/wp-social-pro-documentation/the-ik-facebook-plugin/
– Jetpack by WordPress.com 2.9.2
http://wordpress.org/extend/plugins/jetpack/
– jQuery Vertical Accordion Menu 3.1.2
http://www.designchemical.com/blog/index.php/wordpress-plugins/wordpress-plugin-jquery-vertical-accordion-menu-widget/
– liteAccordion Skin for WP slider 0.1
– MaxButtons 1.23.0
http://maxbuttons.com
– mb.miniAudioPlayer 1.5.1
http://wordpress.org/extend/plugins/wp-miniaudioplayer/
– P3 (Plugin Performance Profiler) 1.4.1
http://support.godaddy.com/godaddy/wordpress-p3-plugin/
– Page Links To 2.9.3
http://txfx.net/wordpress-plugins/page-links-to/
– Posts in Page 1.2.4
http://www.ivycat.com/wordpress/wordpress-plugins/posts-in-page/
– Simple Breaks 3.0.0
http://www.hitreach.co.uk/wordpress-plugins/simple-breaks/
– Social Media Widget 4.0.2
http://wordpress.org/extend/plugins/social-media-widget/
– Spacer 1.0
– TablePress 1.3
http://tablepress.org/
– TW Recent Posts Widget 1.0.3
http://vuckovic.biz/wordpress-plugins/tw-recent-posts-widget
– Ultimate Category Excluder 0.96
http://www.planetmike.com/plugins/ultimate-category-excluder/
– Use Shortcodes in Sidebar Widgets 1.0
http://softcontent.eu/use-shortcodes-in-sidebar-widgets.html
– Velvet Blues Update URLs 3.2.2
http://www.velvetblues.com/web-development-blog/wordpress-plugin-update-urls/
– Vice Versa 2.2.3
http://jasonlau.biz
– Widgets on Pages 0.0.12
http://gingerbreaddesign.co.uk/wordpress/plugins/widgets-on-pages.php
– WordPress SEO 1.5.2.5
https://yoast.com/wordpress/plugins/seo/#utm_source=wpadmin&utm_medium=plugin&utm_campaign=wpseoplugin
– WordPress Slider Plugin Pro 0.5
http://muneeb.me/wordpress-slider-plugin/
– Wufoo Shortcode Plugin 1.42
– YITH Maintenance Mode 1.1.1
http://yithemes.com/
—– End System Info —–