I had done a mistake in configuring the query parameters. Apparently, one has to remove any preexisting '?' from the query when appending a new query to a UriBuilder.Query property. So I basically had to do a Substring(1) on the query like they describe here:
That's a pretty confusing error message and a pretty weird property...
I'm actually trying to utilize the RestClient you wrote and I'm running into this same error ("Could not create SSL/TLS secure channel.") uploading apps. I don't have this problem uploading extensions.
Is it a fair conclusion it must be the endpoint that is not set correctly? I don't have this issue with any other endpoint, just the upload app one.
Here's my endpoint:
string uploadAppEndpoint = "/qrs/app/upload?name=" + Uri.EscapeDataString(System.IO.Path.GetFileName(filename));