Blazor App No Longer Compiles

The stack trace is from Hot Reload session startup, so first separate a Release compilation failure from an IDE launch failure. From the project directory, run dotnet build --configuration Release and capture its output. If that succeeds, the compiler can build Release and the next investigation should focus on Visual Studio's launch tooling. Disable Hot Reload temporarily in its settings, or set hotReloadEnabled to false in the relevant launchSettings.json profile, then retry the same launch. Microsoft documents that profile setting; this is an isolation test, not a confirmed fix for your exception. If the command-line build fails too, investigate its first actual build error instead. Also copy the complete product and version information from Help, About, since the stated product name and version need clarification. Report the issue with your minimal new-project reproduction, target framework, SDK version and both test results.