[TYPO3-windows] Typo3 Backend 500 Server Error After Upgrading toT3 4.3.1 on Windows 2008 IIS7 (Pim Broens - Redkiwi)

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I've actually had some similar problems with my installation. Might be
> the same issue. I believe that there might be some .htaccess files that
> aren't escaped (_.htaccess) in the TYPO3 folders which make for a bad
> rewrite that isn't allowed. And in turn makes your server go 500 on you.
> Maybe doing a search for those and escaping them might help you a bit?
>
> Good luck!
>
> Kind regards,
>
> Pim Broens
> Redkiwi
>
>
There are no .htaccess files in any of the main Typo3 folders, so I don't
think it due to that.  We are using Isapi Rewrite program which has the
rewrite rules stored in the main httpd.conf file, and it can also read
.htaccess files if they were present, but I couldn't see any.

What is the next line of troubleshooting?