For the benefit of others, I got a fix elsewhere as follows:

Problem is a corrupt ID.

Fix:

Create new OE ID and import all folders, add-bk, accts., etc., then delete old ID once you're sure the new one is working OK.

Also: disable anti-virus auto-check on email... which is OK providing you have some other safety checker facility in place to stop downloads of infected mails (as I do... SpamKiller) or read your mail on the web before downloading.

Also: compact everything weekly, remembering never to shut OE using Task Mgr while it's doing it.

Thanks, folks.