There is no option to downgrade vCenter server. I would say wait for couple of months maybe this will be sorted in next release of 6.5.
As said in the KB the resolution is to upgrade to 6.7 U1. If your backup vendor does not support this you have to wait until this is supported by your backup vendor.
I suggest to contact your backup vendor and ask when they expect compatibility for 6.7 U1.
Yes, should be any day now I hope. Unitrends says mid-Q4...