still having issue with “Invalid OAuth State”

This issue was addresses a couple years ago, and was marked “resolved”, but I am still having an issue with the “Invalid OAuth State” error. I have followed the instructions closely, private/incognito windows in Firefox, safari, and chrome, but this has not resolved the issue. My site will sent new account emails, but not order notification emails. And when I authorize the plugin to integrate, it will say that it is configured, but if you refresh the page, the Invalid OAuth State error will pop up. What else am I missing?

The page I need help with: [log in to see the link]