Same, 10+ people have reported to me that the iOS app no longer works for them, including myself.
I had the same error and its was because of a missing websockets config in my reverse proxy (haproxy).
haproxy.cfg (frontend) :
acl hdr_connection_upgrade hdr(Connection) -i Upgrade
acl hdr_upgrade_websocket hdr(Upgrade) -i WebSocket
acl host_emby hdr_sub(host) -i emby.<mydomain>