Here's the solution to this problem, from the Success Centre:
Whilst this is a somewhat old question, I hope the above helps others who come across this problem!
That knowledge article doesn't make any sense...
If your IPAM server is Domain1\IPAM, and your adding a DHCP server: Domain2\DHCP, how does creating a local user on IPAM fix this? Surely IPAM\username isn't going to work when adding Domain2\DHCP because that authentication is still going to go to Domain2\DC...
Even if you were to create a domain account for this purpose, Domain1\Username will never be able to logon to Domain2.