3.9 introduce an updated visual editor (using the latest version of tinyMCE). This has caused a lot of plugins and themes to have issues as they used deprecated code to “tie-in” to the visual editor. So you’ll want to track down which theme or plugin is causing the issue.
To do this, disable all plugins and custom themes. Slowly re-enable them testing them one at a time until the issue appears. Inform the developer of said plugin/theme that was last enabled.
If this doesn’t find or solve your issue, consider doing a manual re-install and follow these steps:
1) First make a backup of your database and files.
2) Delete all the WordPress files off your server except the wp-config.php file and the wp-content/uploads folder (yes, we’re deleting your themes folder and plugins folder with everything in them).
3) Now download the latest version of WordPress, unzip it, and then upload all the files to your server (obviously don’t overwrite wp-config.php or the wp-content/uploads folder).
4) Point your browser at your site. You should have a working default WordPress installation at this point.
5) Now upload your custom theme and enable it. Does everything work? If not, the theme is your problem and you should contact the developer for support.
6) If your custom theme works, it’s time to slowly add in plugins to the mix. Add in and enable plugins one by one. Test your site after enabling a plugin to see if everything works. If any errors show up, note which was the last plugin enabled and contact the plugin developer. Disable that plugin until a fix has been released or you can find a similar plugin that works with your configuration.
At the end of this you’ll hopefully have a stable and working WordPress site.
