After upgrade to Joomla 3.7.0 an error was displayed when saving an article in the frontend. The error message was “Warning. Empty solution not allowed”.
The error message is a bit confusing, not giving any real hint what it is about. It turns out it has to do with Captcha. A new feature in Joomla 3.7.0 is the possibility to use Captcha on article editing.
To solve the problem, go to Options -> Articles and select the Editing Layout tab. Make sure “None Selected” is set in the Captcha field and click Save. Do click Save if it is already selected because it seems the setting is missing after the upgrade. By saving, the setting will be added and the problem is solved.