So people, I'm continuing to investigate the issue, but in the meantime if you can't send a PM (it seems to manifest as that you can type it up, but when you hit "send" it doesn't work), then first copy the text you were going to send to the clipboard of your OS, then go down to near the bottom of the page and hit the little "My Cookies" link. Expire your cookies, then log back in. Then you should be able to send your message.
So we know it is a cookie issue, but to fix we might just wait to see if upgrading the board software corrects the issue. I'll be doing that soon.