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We have an ASP.NET MVC 5 App currently running on IIS 10. Everything worked fine for months, however, suddenly 2 days back we started seeing a "403.14" error on random websites. 

403 – Forbidden: Access is denied.
You do not have permission to view this directory or page using the credentials that you supplied.

And what's even strange is that the error goes away if we toggle error page 403 setting in IIS (Show Detailed Error, Show Detailed Local, etc.) only to come back in some different website or even in the same website after a while.

  1. We have a single folder with the compiled MVC code with probably a dozen IIS sites (different domain names) mapped to the same folder.
  2. We have URLRewrite module installed.

  3. The CLR version is .NET 4.0.x

  4. We have tried restarting the IIS service (World Wide web Publishing service)

  5. Also, rebooted the server.
  6. Based on some suggestions, we added runAllManagedModulesForAllRequests to web.config webserver modules config.
  7. The Application pool has Run 32-bit applications enabled.

What could be causing this? The error going away and coming back has confused us even more.

Jack
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  • did you enable directory browsing in your iis site? refer this [link1](https://stackoverflow.com/questions/1741439/asp-mvc-in-iis-7-results-in-http-error-403-14-forbidden) ,[link2](https://support.microsoft.com/en-us/help/942062/http-error-403-14-forbidden-error-when-you-open-an-iis-7-0-webpage) – Jalpa Panchal Apr 08 '20 at 09:57
  • Thank you. I had already checked both of these. Do note, this happened in a setup that was working for several months without issue. After doing a series of things like restarting IIS process, adding more app pools distributing the load, tinkering with web.config just to force IIS to refresh, the matter was resolved. We did add the runAllManagedModulesForAllRequests, however it was not present earlier, and things were working. We still do not know what caused it suddenly. – Jack Apr 08 '20 at 17:02
  • did you check the event viewer log? – Jalpa Panchal Apr 09 '20 at 09:38

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