Yesterday I sent out two PMs to two different people and today awoke to an email notification saying that I had a PM reply waiting.
So I checked my inbox and both replies from both parties were previous email replies they had sent from days ago, not their newest replies.
I now have no record of their new replies and not only that, the forum is now constantly saying I have a new message waiting.
It won’t clear the flag.
Not sure if it’s just me, but thought I’d give a heads-up just in case anyone is experiencing the same thing.
Maybe a PM to the moderators would help ? That is, if they ever receive it ? 
Haven’t sent/got any message lately, but thanks for the heads-up Kevin. I’ll be on the lookout for any similar problems on my inbox…
Strangeness, indeed. I have not experienced such problems, but will post here if I do.
I figured it out.
My mailbox was full, but that was only part of the problem.
Even after I emptied all mailboxes (0% used), it still said I had two messages waiting that I couldn’t retrieve.
My mistake (and PHP’s error also to some degree) is that they have a statement in a small font saying to “Release emails.”
However it’s part of a larger sentence and provides no indication that part of that sentence is actually a link.
I interpreted it as request to clean out my mailboxes, not a link to free up trapped messages.
Interesting. Glad the issue has a cure! 