I tried it again with Chrome, Edge and noticed something... If I am not logged into the site, but go to the link its fine. As soon as I log into the site on Edge and Chrome, it errors out with the error mentioned.
I did notice when you log off from rctech you get a message about cookies being cleared etc. Makes me wonder if something is caching the error page in some manner but on on an end I can control as I have cleared everything from all browsers, etc.
I tried something here, while not logged in went to the thread, hit reply to a post, the reply screen came up and I put in a comment and then logged in, it did the same error but did post my comment?