WSOD for www prefixed site only. Any ideas?

I have advertisers on my site who are testing the www version

Very wise to do so!

Add a www-to-non-www redirect:

https://forum.ait-pro.com/forums/topic-tag/htaccess-redirect-www-to-non-www/
https://www.google.com/search?q=bulletproof+security+www+non-www

Thanks leejosepho. If the advertiser attempts to test the www version, will they still get the white screen? What if they don’t allow redirects?
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I did it and it appeared to conflict with the current wordpress .htaccess code putting it in a redirect loop.

Try placing the www-to-non-www redirect ahead of the WordPress code so that can be resolved before anything ever even reaches WordPress.

What if they don’t allow redirects?

I would assume the issue there would be about not redirecting from one site or page to another and not about a mis-typed URL landing where it should.