white screen after form post

I’ve tried several fill-in form plugins (Ninja, FS Contact Form, Contact Form Manager). On some clients (seems to be dependent on network address not browser type) after the form redirects it goes to a white screen. However if I disable Wordfence it works. This is repeatable. The redirect always works on my home and office PC (any browser) but if I submit from my android phone using chrome, it goes to white screen. Other users have also reported the white screen, but it only affects some users. No IP addresses in the blocked list – at least none of those testing the form.

https://wordpress.org/plugins/wordfence/