Have you just bought it?? And which version of Server??
You might need to go back a version or two..
From the help doc a new line has snuck in:::
New certificatess issued after Oct 2015 (Entrust AdvantageSSL) or Feb 2016 (GeoTrust QuickSSL Premium) are not supported.
Yes, it was a recent purchase, at the start of March, based on the advice FIleMaker was then offering (the cached versions on Google shows that the help doc was quietly edited after I posted this question).
An apology from FileMaker would be nice, for my wasted day of trying to fix the unfixable -- a day that I can't very well bill my client for -- caused by the fact that they didn't notify us that a certificate would no longer be supported until nearly two weeks after it was no longer supported.
Bad show FMI!