OK, after logging this on Bugzilla, the guys noted that the log seemed to show that the failure occurred when trying to send a message in my outbox. When that was deleted, it all worked ! And to my surprise, when I created and sent another message and sent/synched that, it continued to work.
Which leaves me with two thoughts:
This may have been some kind of glitch, perhaps to do with the facts that the message in the outbox ...
a) was a message that was being forwarded, and
b) it had attachments.
Admittedly these may be coincidences but I don't have time at this moment to conduct further experiments.
But naturally, I've updated the bug report and we'll see what the team makes of this.
Others having problems may like to see if this gives them any leverage ;-)