Problem is that what you have in post 4485169 works for me (cut&pasted, no change whatsoever):
- as long as I do not add something like ?w=http://www.example.com as a parameter to the URL, it does create a 404 result with"oops" as content.
- With that parameter is gives a 200 result that is a blank page.
It all depends if what you have in the referenced post is your real test case.
If it is not: try it for yourself (without other things in the php).
If it is, then digging deeper isn't going to come on it's own, and it's unlikely others will find it for you.
You'll have to make sure the php is evaluated, dig further in access and error logs, run it command line, look at versions, ...