I have hundreds of Darktable DNG files that have been rejected by InSync. I’m not 100% clear if Darktable actually modifies the DNG, or just uses the DNG.xmp file to store non-destructive changes. I think the latter. Which poses the question “why would Insync reject files produced by a Pentax K3; one of the best DSLRs you can get?”. I seems I can’t upload a file to share with you, and I can’t give it to you via Drive as it won’t upload; but I’m happy to email you.
Hi @stevebarnes have you emailed your issue to support@insynchq.com? 
If not, please include this link and the logs.
Thank you!