Neil,
not sure which certificate are you talking about. But looks like your installed certificate does not include the intermediate CA from your signing CA. Your installed certificate must include all certificate from all sub CAs.
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Make sure that the CRT file has the full certificate chain up to a trusted root CA.
Usually you get the certificate chain from the signing CA. Sometimes it split into separate files. If the signed certificate and the trust chain are in separate files, use a text editor to combine them into one file. Make sure the server certificate is at the top of the CRT file.
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regards
Wolfgang