Signalr Connection is getting disconnected immediately

I have .NET Web API which hosts the Signalr Hub on Server1. Web API app getting the signalr connection request from Asp.NET application hosted on different server Server2, via IIS rewrite proxy. As in Asp.NET webapp we has embedded Angular 6 app inside the iframe, which is sending request for connection via proxy .

But as soon connection is established, it is being disconnected after 2-3 seconds. Then subsequent requests to Signalr, where we try to call server methods failed as client is disconnected.

Error in transport logs:

**SignalR.Transports.TransportHeartBeat Verbose: 0 : 0c6a1267-4ffc-4b96-8c6e-1c79f805e4d4 is dead
SignalR.Transports.TransportHeartBeat Information: 0 : Removing connection 0c6a1267-4ffc-4b96-8c6e-1c79f805e4d4**

Error in FRT for method invocation from client side.

106. GENERAL_READ_ENTITY_END BytesReceived="0", ErrorCode="The I/O operation has been aborted because of either a thread exit or an application request.
 (0x800703e3)"
176. GENERAL_NOT_SEND_CUSTOM_ERROR Reason="SETSTATUS_TRYSKIP"
183. GENERAL_FLUSH_RESPONSE_END BytesSent="0", ErrorCode="The parameter is incorrect.
 (0x80070057)"

How can I figure out what is causing the issue?

I have enabled FRT in IIS and Signalr diagnostics in web.config as well, but no reason found for above issue. CORS is already enabled.