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If you can confirm that it works for other files on the same server then you can rule out anyting on the server itself. The one server setting that you did not mention is where you set up FMS to allow "FileMaker and external accounts". But again: if it works for other files then you are ok there.
Do the other files use the same group? If not then triple-check the user's membership in that group.
That leaves something in the file. Perhaps a typo in the group name?
Are the user's machines part of an AD/OD domain?
Thanks for the help. I checked everything you suggested, and could find nothing wrong. I then realised that the only difference between this "problem file" and others was that the good ones are all in sub-folders within the Filemaker Server/Data/Databases folder. So I removed the file, and re-uploaded it to a sub-folder, opened it, and suddenly all is well.
I don't see why moving it to a sub-folder should make any difference, so I can only assume that there was a problem with the syncing of the Active Directory between servers, and that the issue rectified itself over time - I literally changed nothing else.
Anyway, after 2 hours of faffing about and scratching my head, all is fine. But I wish I understood why! Thanks again for your input.