I'm having the same "Incorrect Content-Type" error. The problem appears to stem from the browser POST-ing to /checkout/paymentmethod but then being GET-redirected to /checkout/paymentinfo. Manually changing the action of the form to /checkout/paymentinfo solves the error and allows the payment to proceed.
Only way process the order with Authorize.net is use One Page Checkout. Regular checkout getting error. Short message Incorrect Content-Type: Full message System.InvalidOperationException: Incorrect Content-Type: at Microsoft.AspNetCore.Http.Features.FormFeature.ReadForm() at Microsoft.AspNetCore.Http.Internal.DefaultHttpRequest.get_Form() at Nop.Plugin.Payments.AuthorizeNet.Components.AuthorizeNetViewComponent.Invoke() at lambda_method(Closure , Object , Object[] ) at Microsoft.Extensions.Internal.ObjectMethodExecutor.Execute(Object target, Object[] parameters) at Microsoft.AspNetCore.Mvc.ViewComponents.DefaultViewComponentInvoker.InvokeSyncCore(ObjectMethodExecutor executor, ViewComponentContext context) .....