In all browsers?
Safari and Chrome both have the same problem
OK so the way to "resolve this" is to disable secure progressive downloads of container content. I don't understand why that is a problem as we are using Geotrust QuickSSL Premium which is on the list of acceptable certificates for FileMaker Server 14.
I expect the hosting provider may not have enabled the port required for progressive downloads.
At some point will also try that on our own server and will update here.