Make sure your startup project is set to Web.Server (ASP.NET Host).
This kind of error appears when you try to start the Web.Client directly.
blazor.webassembly.js:1 crit: Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
Unhandled exception rendering component: Could not load settings from '_configuration/Havit.Bety2.Web.Client'
Error: Could not load settings from '_configuration/Havit.Bety2.Web.Client'
at Function.createUserManager (http://localhost:59799/_content/Microsoft.AspNetCore.Components.WebAssembly.Authentication/AuthenticationService.js:1:5893)
at async Function.initializeCore (http://localhost:59799/_content/Microsoft.AspNetCore.Components.WebAssembly.Authentication/AuthenticationService.js:1:5027)
Microsoft.JSInterop.JSException: Could not load settings from '_configuration/Havit.Bety2.Web.Client'
Error: Could not load settings from '_configuration/Havit.Bety2.Web.Client'
HAVIT Knowledge Base 
Oh. it is interesting, i know what is the problem, but What is the solution?
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In Visual Studio set the startup project to ASP.NET Host (server project).
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