If enabled, more detailed error messages will be shown on every error that occurs, otherwise a simple generic error is shown.
To enable use the debug mode edit config/general.php and set 'debug' => true,.
If you get a "Opps! Unexpected error." error message, you can use the browser dev tools to see the actual errors and response from server (make sure the debug mode is enabled).
In Google Chrome, right-click and select Inspect Element, or use the keyboard shortcut: Ctrl+Shift+I (or Cmd+Opt+I on Mac).
Navigate to the Console tab to see potential JavaScript errors and the Network tab to see the HTTP requests (make sure Record Network Log is on).