The similar problem we ran in to seems to be centered around the fact that everyone else uses HEX formatted keys (64 characters), while Cisco requires 32 character ASCII keys.
We tested by making an ASCII key and that works, but is DEFINITELY sub-optimal in that our keyed NTP provider would need to issue two sets of keys for each of SHA1 and SHA256 for every customer - one SHA1 and SHA2 for Cisco nodes and one SHA1 and SHA2 for every other vendors' nodes.
It would be great if Cisco would support HEX keys, please ... (IOS-XE 17.09.04a)
