I don['t believe it matters whether they're on the same server or different servers - i've had up to 5 mail servers, and at times have used the registration server as the mail server until a time that I set up a new server just for admin purposes.
From the two log entries you provided, it appears that 1) some document is not found and 2) you can find the ID Vault.
Do you receive any other errors?
Are you setting the Resigstrationserver property?
Do you have a vault configured? Is in located on the same mailserver?
It might help to post that portion of your agent where you're setting the properties and subsequently calling the 'RegisterNewUser' method.
You also might search the Note 6.0 forum as well as the 8.5 forum for sample code, as well, because there have been a number of posts over the years for issues surrounding this process.
Feedback response number WEBBB37RJP created by Mark Taylor on 07/31/2018