Error Domain=NSPOSIXErrorDomain Code=100 "Protocol error"
This was a tricky one at least for me for an entire day ð . I figure out at the end how to solve it, it was an HTTP protocol version issue with the server.
We were using IIS to connect to a development API and by default IIS if I remember correctly above 10 use HTTP 2.0 if both the client and the server support it. Also, our server was using an SSL certificate for the development API so I think this was the main issue combined with HTTP 2.0. Disabling the use of HTTP 2.0 for the IIS in the development API fixes the issue.
So my advice is to try to monitor the incoming request to see which kind of protocol is using the request in the server, you can do it with Charles or any another HTTP monitor tool.
I hope this helps someone else with the same problem.
I got exact same error as yours in Cocoa with foundation class URLSession. After hours debugging the issue lies in the HTTP request body.
You should really try to dump the HTTP request/response body to see if there are some malformed fields. For example, Content-Length and Content-Type are right or missing? In my experience if these required(fundamental) headers are malformed, it may not work depending on your OS or other intermediate network accept(e.g. proxy, gateway, server, etc.)
My fault is misplacing function params in the method URLRequest.addValue("application/x-www-form-urlencoded", forHTTPHeaderField: "Content-Type"), which ends up with a wrong Content-Type HTTP field.
However, it works in macOS 10.12 but not 12.11 so you should makes sure your HTTP Request body is not malformed.
Hope it helps.
Form your sample code, I guess the encoding: JSONEncoding.default is wrong. Since an HTTP GET method has NO body, a strict/not-robust network component would reject/not understand it.
What your goal is set the Accept: application/json in the request header, however it's not required if you're sure the response body type.
I got this error with Apache sending an upgrade header. Here is a link to Apache's bugzilla discussion on the issue.
Fixed it by adding Header unset Upgrade to host config in Apache.