We get this error:
3,2,14,(TESTMODE) The referrer, relay response or receipt link URL is invalid.,000000,P,0,,,25.02,CC,auth_capture,,,,,,,,,,,,,,,,,,,,,,,,,,10639226DFAA135546EABD52127F585D,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,01,14

We have added the URL in the Payment settings of the control panel to the Authorize.net settings.
(https://example.com/themes/third_party/cartthrob/lib/extload.php/authorize_net_dpm)

We also tried it without the URL in there, http/https, and still get the error. It was working ok up until we ran a test after making some jQuery and validation tweaks to the checkout process. We are using 2.1.5 and the DPM gateway that comes with it.

(then to test other methods I used the standard Authorize option. It had all the CC fields on the checkout page and did submit successfully. Is it supposed to keep my on my site?).

Is it supposed to keep my on my site?)

Yes.

We also tried it without the URL in there, http/https, and still get the error. It was working ok up until we ran a test after making some jQuery and validation tweaks to the checkout process. We are using 2.1.5 and the DPM gateway that comes with it.

I’ll have to take a look the DPM gateway is something I basically built for you guys (if I recall correctly). If it’s getting errors, I’ll have to review directly. Can you email me the EE / FTP access for this? I can test in more detail.