Clear out your old PMs, as there is now a quota or limit
Maybe there always was a limit, and I had already hit it, and simply hadn't gotten that next one that got rejected and got an alert sent my way.
Not that I knew of, but it's possible.
But at least as of now, as the alert I received explained, there is such a limit, and I didn't get a PM because I was over it (the alert mentioned the sender).
Trying to let the intended sender know I was clear to receive, I found HE was over the limit. Maybe we all are.
Looking at my limited list, I did not see any indication what that limit was (it did say I had 18 unread or so, which was correct on number but wrong as to my having not read them-- I had).
There is some chance that simply going through and reading them to change that status would allow for more receivable messages, but I can't do that experiment as I already deleted the oldest ones to make room.