Broken API?

[PFM]

I have two IP2M-841W cameras running on an internal network. I have written a web-based interface (.aspx with C# code-behind) to get snapshots as well as operate a variety of camera functions, including PTZ. After updating both cameras to the latest firmware (2.520.AC00.18.R) my interface stopped functioning.

I found, through the forums, that despite the latest SDK documentation, the camera no longer accepts basic authentication. I re-programmed my code to use digest in all http requests and I am now able to get some functions working, including the snapshot. However, I am not able to use my PTZ controls, specifically:

http://cameraIP:port/cgi-bin/ptz.cgi?action=start&channel=1&code=PositionABS&arg1=0&arg2=0&arg3=1&arg4=1

Using a browser I get: Server internal error! as a response, for both cameras.

Please advise what, if any, http commands I can now use for this function. Should I presume that the API has changed? Do you have new documentation, other than what is on the Amcrest website?