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app.UseCors(CorsOptions.AllowAll)

One thing that caught me out after enabling the middleware is that IIS was intercepting pre-flight requests. To ensure ASP.NET handles OPTION requests, add the following in web.config:

<system.webServer>
 <handlers>
   <remove name="ExtensionlessUrlHandler-Integrated-4.0" />
   <remove name="OPTIONSVerbHandler" />
   <remove name="TRACEVerbHandler" />
   <add name="ExtensionlessUrlHandler-Integrated-4.0" path="*." verb="*" type="System.Web.Handlers.TransferRequestHandler" preCondition="integratedMode,runtimeVersionv4.0" />
 </handlers>
</system.webServer>