Can't connect on-premise users to exisiting Office 365 users

[JapeMincer]

Hi Guys,

We've recently taken over the IT support of a customer from another IT company.  They had Office 365 synced to their on-premise DC of the previous IT company.  The directory sync has been stopped and AD Connect removed from the previous DC.  The Office 365 accounts now show as 'Cloud'.

I have configured Azure AD Connect on our DC and ran through the setup, which I believe to be correct, however I just can't get the on-premise accounts to connect with the existing O365 accounts.  I get the following error "AttributeValueMustBeUnique".  I have verified that the UPN's match as does the primary SMTP but the issue remains.  Any new accounts created in the on-premise AD sync without issue. 

Kind Regards,

James

[#2 · Accepted solution]

Hey James. Have you nulled the objectid? See this article for reference. Talks about it in immutable section.

https://www.google.com/amp/s/365lab.net/2014/01/26/office-365-migrating-dirsync-to-new-ad-domain/amp...

[#3]

Thank you very much Chris, that seems to have done it! 

[#4]

Cool glad it worked! Funny I was just looking this up to do it myself last night and caught your post and remembered that step so figured it might be it :)