Okta: Unable to obtain configuration from: 'System.String'

I'm building an app using Okta for authentication, but the API requests will always return a 500 error. I use @okta/okta-react to get the access token, but when sending in the Authorization header (Bearer {accessToken}) the server is unable to handle it.

The server replies with the following error for every request.

Microsoft.AspNetCore.Authentication.JwtBearer.JwtBearerHandler[3]
      Exception occurred while processing message.
System.InvalidOperationException: IDX20803: Unable to obtain configuration from: 'System.String'.
 ---> System.IO.IOException: IDX20807: Unable to retrieve document from: 'System.String'. HttpResponseMessage: 'System.Net.Http.HttpResponseMessage', HttpResponseMessage.Content: 'System.String'.
   at Microsoft.IdentityModel.Protocols.HttpDocumentRetriever.GetDocumentAsync(String address, CancellationToken cancel)
   at Microsoft.IdentityModel.Protocols.OpenIdConnect.OpenIdConnectConfigurationRetriever.GetAsync(String address, IDocumentRetriever retriever, CancellationToken cancel)
   at Microsoft.IdentityModel.Protocols.ConfigurationManager`1.GetConfigurationAsync(CancellationToken cancel)
   --- End of inner exception stack trace ---
   at Microsoft.IdentityModel.Protocols.ConfigurationManager`1.GetConfigurationAsync(CancellationToken cancel)
   at Microsoft.AspNetCore.Authentication.JwtBearer.JwtBearerHandler.HandleAuthenticateAsync()

I'm using the Okta.AspNetCore package for the configuration. My startup class has

public void ConfigureServices(IServiceCollection services)
{

services.AddAuthentication(options =>
    {
        options.DefaultAuthenticateScheme = OktaDefaults.ApiAuthenticationScheme;
        options.DefaultChallengeScheme = OktaDefaults.ApiAuthenticationScheme;
        options.DefaultSignInScheme = OktaDefaults.ApiAuthenticationScheme;
    })
    .AddOktaWebApi(new OktaWebApiOptions()
    {
        OktaDomain = Configuration["Okta:Authority"],
    });

services.AddAuthorization();
services.AddControllersWithViews();

...
}

and

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.UseRouting();
    app.UseAuthentication();
    app.UseAuthorization();
    app.UseEndpoints(endpoints => ...
}

I've done a lot of searching around for answers, but most of them are similar, but not the same issue. Any help would be appreciated!