Things I'd try next (since it sounds like it will be an on-going annoyance, to say the least):
1) Try posting an anonymous reply using an otherwise identical setup.
2) Try your account on someone else's copy of Safari. You could do this by loaning someone you trust your password or getting someone to loan you their computer.
3) See if POSTing is part of the problem. Change www.perlmonks.org (or whatever) in your hosts file to localhost and write a quicky web page (or web server) that will deliver a copy of a PM page, accept your reply submission, and deliver the copy of what PM gives back. If that doesn't trigger the problem, then consider sacrificing a few chickens. If it does trigger the problem, then you've built a perfect lab for figuring out exactly which changes prevent the problem.
As for things that changed at PM in the last while, I didn't have too much trouble finding pages from a few weeks or months ago in my browser cache. My files won't match your setup, but you might also be able to find such.
Then, of course, there are the standard things that I often call "voodoo" debugging, that is, changing any number of things that may or may not be related hoping to make the problem go away (user settings, upgrade the browser, flush cache [though I might actually try that one at some point], ...), but I usually prefer to identify the problem...