Detected a connection attempt to an ASP.NET SignalR Server. This client only supports connecting to an ASP.NET Core SignalR Server

Have you solved your issue? As mentioned by JohnB, this is likely an issue of a core client trying to access a .NET framework hub.

If you're trying to connect to a .NET framework hub, you're gonna want to use the signalr package.

Otherwise, if you're hub is a .NET core application, you'll instead want to use @aspnet/signalr.

As Stefano and Ibanez commented is a problem with "versions".

The client of SignalR you are using is able to connect to ASPNET Core but not ASPNET server like error mentioned.

If you know ASPNET Core is a split from .Net Framework (CLR) based for multiplatform.

Then you have two options over this scenario.

First you can change your client side if you desire to continue using ASPNET server side. Then change the library you using to one that supports ASPNET. Give a look: SIGNALR - ASPNET vs ASPNET Core

Second you can change your server side and use ASPNET Core for SignalR, as a microservice, for example. Then continue implementing your client with ASPNET Core SignalR library.