Site Not Verified

Hello,

I have published the verification page as indicated, I can verify it is
correctly published by simply copying and pasting the mentioned url to
my browser, the page shows correctly, but the verification engine keeps
returning me the same message:

NOT VERIFIED

We've detected that your 404 (file not found) error page returns a
status of 200 (OK) in the header.

What can I do?

Thanks,