Ok, looks like no one is checking this thread any more.
Thankfully, I found the solution to my particular problem. And since I HATE it when people post their problems on a forum and then never return to share the solution once they find it, I thought I would return here in case anyone finds this forum in a google search.
In my particular scenario, I managed to figure out that the problem wasn’t anything to do with the installation of WordPress, the theme, or plugins, etc. To come to this conclusion, I set up a brand new WP installation elsewhere on my server. When this new installation still had the same problem, I knew it was most likely something to do with the server. So I emailed my host. I suggest you do the same. Here’s what my host wrote back:
“Those are security errors, meaning the server has detected a common exploit technique in what you’re posting back to the server, so its blocking this.. I think recently a update in software has made more false positives like yours so im still trying to tweak them. I’ve disabled this now on your account, give it a go now and let me know the result.”
Whatever security setting my host disabled managed to fix my problem for good. ??
I hope the above helps anyone else in a similar predicament. To retireate, in my case it had NOTHING to do with the WP installation, its version number, its theme, or plugins. It was strictly to do with some security settings on the server controlled by the host.