I think I might have worked out what is going on.

Now that I have an ADSL line I'm tending to leave IE running all the time (in the past I would have closed IE when I had disconnected). I think that the WWWThreads software, that the board uses, stores info on how recently you read each thread in temporary cookies. This would explain why I might have seen a change in the way things are working.

It doesn't explain why WWWThreads is getting confused, but at least it gives me a work-around (closing and re-opening my browser before starting a BBS session).

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Unit serial number 47 (was 330 in the queue)...