Browser shows uncostomized 500 error page while 404 on log

I am using iis 8.5 as web seerver to publish apps from JBOSS, which is my WAS.

I am facing a security problem where my website does not show my custom 404 error page, but shows 500 error (which I have not customized)

when I try to access not existing directory

while error code 404 is recorded in my server log file.

I believe I have correctly set up my error config, but very strangely this does not work.

Has anyone experienced this?

Hope we can share some solutions.  THNX