A bug can occur in Joomla when Google recaptcha is being loaded in multiple places on a site. The error message “Empty Solution Not Allowed,” most commonly occurs when trying to edit content through frontend editing. The error shows after hitting the SAVE button. The easiest solution is to simply disable the recaptcha feature from frontend editing.
On the back end of your site, go to Content > Articles > Options.
Next, click on the Editing Layout tab, and change the setting “Allow Captcha on Submit” to NONE.