I don't know if it is a solution but here is what has happened.
On Sunday, my site was not loading anymore. I reached out to my host who couldn't find anything. It might have been a glitch or a hiccup but nothing serious. However, while searching, the host pointed that as i am using Cloudflare, I should have the side Proxied, which it wasn't, so I changed it to DNS.
Looking at the timeline, I started having issues (and reports from members) on Monday morning. I never made the connection with the Cloudflare settings.
Even though disabling the Wordpress SSO plugin fixed the issue "temporarily", it would not let the users log in with their WP login, so it was not a solution. Re-enabling it SEEMED to fix the initial issue, but it was short-lived and it was intermittent for some members and not others. While still waiting for Stuart to reply to the help request, I remembered that "Proxied" setting that was changed just before the problems arrived.
So, I went back to Cloudflare, and changed the setting back to "not-proxied" and it looks like everything is back to normal; people can post, respond, react, etc.
So I am sharing this in case it could help others.