Forbidden message after registration

[scarycanary]

Every now and again one of my users informs me that he is getting a forbidden message after trying to register. I have tried to replicate this on phones, tablets, different browsers and I never have a problem signing up to my forum.

Has anybody got a clue as to what might be happening every once in a while to people trying to sign up?

Running v1.7.0 on CENTOS 7 with NGINX reverse proxy.