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To answer my own question
...
1. I was telling people the website address with WWW (e.g. www.xoops.org)
2. In mainfile.php the XOOPS_ROOT constant had no WWW (e.g. xoops.org)
The discrepancy between 1 and 2 above caused the problem.
I've changed mainfile.php, and will get the bloke that owns the server to change it so that it always goes to the WWW version of the site name.
Hope this is useful for someone else...