Edit: I have fixed my issue, turns out our application loads our maps in a couple of different areas and in one of the areas we were setting the SecurityProtocol, but not in the other.
I'm having this same issue.
Our application is built on .Net 4.5 and I've tried every SecurityProtocol (Tls12, Tls, Tls11, Ssl3), and none of them work. I found online somebody said to true ServicePointManager.Expect100Continue = true; but that doesn't help either.
We're using this address: public/NFHLWMS (MapServer)
I also tried this one
None of it works.
Thanks for any help anybody can provide.
I actually had the same problem and just found that issue went away with the newly released version of ArcGIS for AutoCAD 370. Now it works with no problem!
This is the URL I'm using