Have you tried:
– deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).
– switching to the default theme to rule out any theme-specific problems.
– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
– re-uploading all files & folders – except the wp-content folder and wp-config.php & root .htaccess files – from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones.
Same problem w/ visual editor after upgrade to 3.8.1, but only in Firefox, not in Chrome. Found out it was because the visual editor needs Java/Javascript, and my FF add-on of QuickJava had toggled off J/JS when I did the upgrade. Why? God knows! When I switched J/JS back on, visual editor worked fine. Found that only after trying all of the above steps.
Maybe this will help someone…
