The OpenShift Origin WordPress “quick start” application has an error described in this article. The workaround is simply to use a different URL to access the WordPress setup page.
The error occurs in my testing when i opt for scaling. Therefore i believe the error relates to a combination of DNS and HAProxy.
The error sends you to the page:
So you can account this by going to the correct page:
I am not sure why it happens but this workaround should get you moving past the error.