Don't think this is a bug per se, but when I tried to send a PM to a buyer this afternoon, it failed with this:

It did work this morning. There was nothing unusual in the message, all text. I originally had a link in there, but when I removed it, it made no difference. I got the same error just trying to "Save" it as a draft.
(I haven't tried it again, as I have no reason now to PM anyone!)