After windows update the IIS7.5 has no permission to read config file

I have a webserver on localhost, which I use for issue tracking, testing, etc... It has an error now (after many years of proper working)

HTTP-hiba: 500.19 - Internal Server Error
A kért laphoz való hozzáférés sikertelen, mert a laphoz tartozó konfigurációs adatok érvénytelenek.
Részletes hibaadatok
Modul   IIS Web Core
Értesítés   Ismeretlen
Kezelő  Még nincs meghatározva
Hibakód 0x80070005
Konfigurációs hiba  A konfigurációs fájl nem olvasható, mert nincs megfelelő engedélye.
Konfigurációs fájl  \\?\D:\creation\software developer\projects\webissues\document root\web.config

sorry for the Hungarian text, I don't have the exact translation

The error is about: no permission to read the web.config file.

I did nothing with the server last week, but I installed some windows update, so I guess one of them did some black magic. How can I solve this once and for all?

I have many different pages, so I would appreciate a solution which don't requires the reconfiguration of these pages one by one.

edit

I added permission to IIS_IUSRS to read and write the folder of every page, but it did not make any difference. :S

I added permissions to both IIS_IUSRS and IUSR by properties / security tab / edit / add. If you leave out one of them, it won't work.

The permission change might give you access denied by multiple files, you should remove those, if possible, or move them to a different folder until the modifications are done. If you have many of these alerts, it might stop the process.