Sorted now, with extensive help from Tech Support, but I post it here in
case it happens to anyone else:
Client had set the hostname for the machine in /etc/hosts (incorrectly)
in uppercase, with an alias (correctly) in lower case. AAAAaaaarrggghhh!
akent@cix.compulink.co.uk (Andy Kent)
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Freelance Informix Database Specialist,
Redland, Bristol, England