Websockets not working on iOS and Safari - OSSStatus Error 9837

I had the same problem and managed to fix (or rather work around) it by additionally allowing TLSv1.2 - so there seems to be some problem with websockets and TLSv1.3 in current versions of Safari.

While not a solution for production environments, this can be resolved in Safari by enabling the experimental feature NSURLSession WebSocket. To do this go to Develop > Experimental Features and make sure NSURLSession WebSocket is checked.