When I set IIS web server, I met multiple times about HTTP Error 500.21 – Internal Server Error.
I can not remember what the error screen exactly was, next time I must copy the screen.
I always forget how I got big help using a fast solution:
JUST RUN the command aspnet_regiis.exe –i
Very simple but you have to know it is not always working, but it is the one of you can choose to try.