Subfolders for the web.config file are not allowed. The web.config have to put in root-folder and not
in a sub-folder, e.g. for testing a secondary test-application.
It doesn't works in many cases like handlers-add declaration, etc.
... but it's not like it shows. Now you put on the specific error messages with set <customErrors mode="Off"> attribute in the web.config (seen in the last screenshot). It will shows more information about an error if no specific information is displayed.
Now the first error is disapeared, but there is no other special hint for wors configuration. Now you
had to check the headline error message. Its an directory error displayed!
In my case it helps as well to put the ASP.Net web.config file out of a subdirectory and put in the root directory from the server configuration file-system.