Are you sure your not cookie blocking Addons by installing such can block posting from happening since they can't track your activity on their site to verify the poster.

I don't have Adblock Plus active on that site.

It's not just adblock it's Addons such as NS/ghostery Addon so not just adblocker that can also cause this issue. So you need to also try with Private Mode window Browser to verify that is the case as well. That will tell if such Addon are causing issue or not.

I tried that - no joy. I tried MS Edge (has no extensions) - also no joy.