i tried this plugin on local and it works perfectly, i tried this plugin on remote site and it doesn't work correctly: when i click on button in the editor and on button on the box to show the code it return this error:
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Can you access yourweb.com/plugins/editors-xtd/highlight_popup.php?
Some servers doesn't allow it (to increase security), so they throw a 500 error.
You have to ask them.