@RIX_IT
I've been informed by XenServer trial support that Lenovo has filed the ticket LEN-605 for this issue, it's probably a private issue tracker though.
They'll let me know once it is fixed.

@johnarvid That error looks like the error that I've been getting... https://xcp-ng.org/forum/topic/4353/delta-backup-failed-invalid-argument

I've switched back to full backups for now.
I'm still investigating and troubleshooting though so no solutions yet...

@nraynaud @olivierlambert Thanks guys!

Thanks for the tip.

I was using node v8.11.4 and had issues compiling during the upgrade then upgraded to the latest stable node version which wasn't v8.x.

I upgraded to v8.16.2 now and it runs fine. Thanks.