To clarify, I'm running 14.0.2

Worked around this by adding intercepting and leap-frogging the 'Preview' iframe functionality based on an article https://adolfi.dev/blog/umbraco-preview/

public class PreviewController : Controller
{
    [HttpGet]
    [Route("umbraco/preview")]
    public IActionResult Index(string id)
    {
        var requestUrl = new Uri(Request.GetDisplayUrl());
        var leftPart = requestUrl.GetLeftPart(UriPartial.Authority);
        return new RedirectResult($"{leftPart}/{id}");
    }
}

and in appsettings.json Umbraco Globals

"ReservedPaths": "~/umbraco/preview"

This also resolved some CORS errors the iframe was introducing, and strips off the &culture=invariant querystring that was causing the error. The drawback is you don't get the different device sizes footer menu.

I guess this is still broken I am getting this!

An unhandled exception occurred while processing the request. CultureNotFoundException: Culture is not supported. (Parameter 'name') invariant is an invalid culture identifier.

System.Globalization.CultureInfo.GetCultureInfo(string name)

Stack Query Cookies Headers Routing 

CultureNotFoundException: Culture is not supported. (Parameter 'name') invariant is an invalid culture identifier.
    System.Globalization.CultureInfo.GetCultureInfo(string name)
    Umbraco.Cms.Web.Common.Localization.DynamicRequestCultureProviderBase.<DetermineProviderCultureResult>b__4_0(StringSegment culture)
    Umbraco.Cms.Web.Common.Localization.DynamicRequestCultureProviderBase.TryAddLocked(IEnumerable<CultureInfo> supportedCultures, IEnumerable<StringSegment> cultures, object locker, Action<StringSegment> addAction)
    Umbraco.Cms.Web.Common.Localization.DynamicRequestCultureProviderBase.DetermineProviderCultureResult(HttpContext httpContext)
    Microsoft.AspNetCore.Localization.RequestLocalizationMiddleware.Invoke(HttpContext context)
    Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
    Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
    StackExchange.Profiling.MiniProfilerMiddleware.Invoke(HttpContext context) in MiniProfilerMiddleware.cs
    Umbraco.Cms.Web.Common.Middleware.UmbracoRequestMiddleware.InvokeAsync(HttpContext context, RequestDelegate next)
    Umbraco.Cms.Web.Common.Middleware.UmbracoRequestMiddleware.InvokeAsync(HttpContext context, RequestDelegate next)
    Microsoft.AspNetCore.Builder.UseMiddlewareExtensions+InterfaceMiddlewareBinder+<>c__DisplayClass2_0+<<CreateMiddleware>b__0>d.MoveNext()
    Umbraco.Cms.Web.Common.Middleware.PreviewAuthenticationMiddleware.InvokeAsync(HttpContext context, RequestDelegate next)
    Microsoft.AspNetCore.Builder.UseMiddlewareExtensions+InterfaceMiddlewareBinder+<>c__DisplayClass2_0+<<CreateMiddleware>b__0>d.MoveNext()
    Umbraco.Cms.Web.Common.Middleware.UmbracoRequestLoggingMiddleware.InvokeAsync(HttpContext context, RequestDelegate next)
    Microsoft.AspNetCore.Builder.UseMiddlewareExtensions+InterfaceMiddlewareBinder+<>c__DisplayClass2_0+<<CreateMiddleware>b__0>d.MoveNext()
    SixLabors.ImageSharp.Web.Middleware.ImageSharpMiddleware.Invoke(HttpContext httpContext, bool retry)
    Umbraco.Forms.Web.HttpModules.ProtectFormUploadRequestsMiddleware.InvokeAsync(HttpContext context, RequestDelegate next)
    Microsoft.AspNetCore.Builder.UseMiddlewareExtensions+InterfaceMiddlewareBinder+<>c__DisplayClass2_0+<<CreateMiddleware>b__0>d.MoveNext()
    Umbraco.Cms.Api.Management.Middleware.BackOfficeAuthorizationInitializationMiddleware.InvokeAsync(HttpContext context, RequestDelegate next)
    Microsoft.AspNetCore.Builder.UseMiddlewareExtensions+InterfaceMiddlewareBinder+<>c__DisplayClass2_0+<<CreateMiddleware>b__0>d.MoveNext()
    Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

At the time of posting this we are on 14.3.1

is working on a reply...