ASP.NET Core 3.0 React app errors: failed to proxy - proxy

Question

(FYI, this is my first Microsoft project of any type and I am not familiar with technologies like proxy servers, etc so my description of the issues may not be the greatest).
I have been working on an ASP.NET Core 3.0 (preview) React app for the past month or so without any issues. Last Friday I started updating the web api to pull data from the database for charting in the UI. This week I picked up where I left off but my project now errors on start up.
Events that occurred since it was last working... I installed and used Fiddler briefly on Friday to run some api calls but then switched over to Postman. Everything was fine and I left everything running (may have closed Fiddler, don't recall). This week I made some database changes (set a multi-column index via the Fluent API) and added more data to the database (SQL Server). I then proceeded to continue working on the web api starting with a request in Postman but it errored. I went to view the app UI but that errored as well.
After two days of googling/troubleshooting I think the most useful error given is the one below. From the error messages, I do not know where port 50158 is coming from. The app loads at https://localhost:44398/. I performed a search for "50158" on my whole computer and only found it in IIS log files. Here are the first 4 lines in each log file. The rest of each file mentions only '44398'.
Successfully registered URL "https://localhost:44398/" for site "MyAwesomeApplication" application "/"
Successfully registered URL "http://localhost:50158/" for site "MyAwesomeApplication" application "/"
Registration completed for site "MyAwesomeApplication"
Request started: "GET" https://localhost:44398/
I found some posts on SO that mention Fiddler causing similar issues to this error, however, they have not helped me resolve the problem.
Some posts on SO mention unchecking "Automatically detect settings" on the Internet Properties > Connections > LAN settings. It is indeed unchecked on my computer.
Some things I have tried:
I created a new project using all the same specifications but get the same errors.
I completely removed Fiddler.
I reverted the latest database changes.
Where is this problem coming from and what do I need to do to fix it?
Please let me know if more (and what) info would be useful. This issue is out of my current knowledgebase.
#
Error message (sorry, not sure how to format it nicely)
An unhandled exception occurred while processing the request.
SocketException: No connection could be made because the target machine actively refused it.
System.Net.Http.ConnectHelper.ConnectAsync(string host, int port, CancellationToken cancellationToken)
HttpRequestException: No connection could be made because the target machine actively refused it.
System.Net.Http.ConnectHelper.ConnectAsync(string host, int port, CancellationToken cancellationToken)
HttpRequestException: Failed to proxy the request to http://localhost:50158/, because the request to the proxy target failed. Check that the proxy target server is running and accepting requests to http://localhost:50158/.
The underlying exception message was 'No connection could be made because the target machine actively refused it.'.Check the InnerException for more details.
Microsoft.AspNetCore.SpaServices.Extensions.Proxy.SpaProxy.PerformProxyRequest(HttpContext context, HttpClient httpClient, Task baseUriTask, CancellationToken applicationStoppingToken, bool proxy404s)
Stack Query Cookies Headers Routing
SocketException: No connection could be made because the target machine actively refused it.
System.Net.Http.ConnectHelper.ConnectAsync(string host, int port, CancellationToken cancellationToken)
Show raw exception details
HttpRequestException: No connection could be made because the target machine actively refused it.
System.Net.Http.ConnectHelper.ConnectAsync(string host, int port, CancellationToken cancellationToken)
System.Threading.Tasks.ValueTask.get_Result()
System.Net.Http.HttpConnectionPool.ConnectAsync(HttpRequestMessage request, bool allowHttp2, CancellationToken cancellationToken)
System.Threading.Tasks.ValueTask.get_Result()
System.Net.Http.HttpConnectionPool.CreateHttp11ConnectionAsync(HttpRequestMessage request, CancellationToken cancellationToken)
System.Threading.Tasks.ValueTask.get_Result()
System.Net.Http.HttpConnectionPool.GetHttpConnectionAsync(HttpRequestMessage request, CancellationToken cancellationToken)
System.Threading.Tasks.ValueTask.get_Result()
System.Net.Http.HttpConnectionPool.SendWithRetryAsync(HttpRequestMessage request, bool doRequestAuth, CancellationToken cancellationToken)
System.Net.Http.HttpClient.FinishSendAsyncUnbuffered(Task sendTask, HttpRequestMessage request, CancellationTokenSource cts, bool disposeCts)
Microsoft.AspNetCore.SpaServices.Extensions.Proxy.SpaProxy.PerformProxyRequest(HttpContext context, HttpClient httpClient, Task baseUriTask, CancellationToken applicationStoppingToken, bool proxy404s)
Show raw exception details
HttpRequestException: Failed to proxy the request to http://localhost:50158/, because the request to the proxy target failed. Check that the proxy target server is running and accepting requests to http://localhost:50158/. The underlying exception message was 'No connection could be made because the target machine actively refused it.'.Check the InnerException for more details.
Microsoft.AspNetCore.SpaServices.Extensions.Proxy.SpaProxy.PerformProxyRequest(HttpContext context, HttpClient httpClient, Task baseUriTask, CancellationToken applicationStoppingToken, bool proxy404s)
Microsoft.AspNetCore.Builder.SpaProxyingExtensions+<>c__DisplayClass2_0+<b__0>d.MoveNext()
Microsoft.AspNetCore.Builder.RouterMiddleware.Invoke(HttpContext httpContext)
IdentityServer4.Hosting.IdentityServerMiddleware.Invoke(HttpContext context, IEndpointRouter router, IUserSession session, IEventService events)
IdentityServer4.Hosting.MutualTlsTokenEndpointMiddleware.Invoke(HttpContext context, IAuthenticationSchemeProvider schemes)
Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
IdentityServer4.Hosting.BaseUrlMiddleware.Invoke(HttpContext context)
Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.MigrationsEndPointMiddleware.Invoke(HttpContext context)
Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.DatabaseErrorPageMiddleware.Invoke(HttpContext httpContext)
Microsoft.AspNetCore.Diagnostics.EntityFrameworkCore.DatabaseErrorPageMiddleware.Invoke(HttpContext httpContext)
Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)
Show raw exception details

Answer 1

For me it had to do with cookie size being too large for development server.
I was working on a React client app served by ASP.NET Core 3.1 with Keycloak as an OP.
Fresh React templates did also suddenly not start anymore.
Clearing cookies made my projects start again.
ref:
https://techcommunity.microsoft.com/t5/iis-support-blog/400-error-in-asp-net-core-project-because-of-8-kb-cookie-limit/bc-p/1308877#M431

Answer 2

I decided to try and roll all the NuGet packages to a previous version (v3.0.0-preview5-19227). My app now functions as expected. Looking at the download page for dotnet-core, it looks like v3.0.0-preview6 was released on 06/12. I started my project way before that so maybe I updated the packages and just don't recall??? Either way, time to make up for 2 days lost.