A Denied HTTP Status not '200' status = 500 message may be received when running an EMV credit card, or attempting to edit an EMV tip. This error indicates that the backend credit card processor is experiencing a server error.
By definition, an HTTP 500 error is a server-side error (more information about HTTP 500 errors here: HTTP 500 Errors Explained).
Since backend processors errors and outages are outside of the control of FOCUSpos or FOCUSpayments, there isn't much we can do to prevent or resolve the problem.
Finding out the status of the backend processor's network may shed some light on the actual problem. Many processors provide somewhat up to date status notifications via their Twitter feeds: