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Upon finding the original issue, I had used the browser "Inspect Element" tool on the post button itself to see if the action was using the HTTP, or HTTPS version (see *screenshot*). If we can ask users which board they are having issues with, as well as to look in their dev tools (Usually Ctrl-Shift-i) in-browser to see if the action on the posting form still says http we can narrow down the issue. I did a search-replace on the database in the pretty-URL cache, which should have taken care of all of these, but there might be a few stragglers hidden in the database causing issues.
I'm 99.9% sure it's a caching issue, but they could also try flushing and renewing DNS might help. https://www.noip.com/support/knowledgebase/how-do-i-flush-my-windows-dns-cache/
Technically, a user can manually change the http to https in the form action in dev-tools and it will work, although not all forum users will want to mess around that deep.
For what it's worth, I cleared my cache, changed browsers, changed computers, and flushed the DNS. None of that worked. However, inspecting the "Post" button, as indicated, showed that it still just had "http" instead of "https." By editing that to add the "s", I was able to post a reply in the Redemption Market.