Windows vCenter and VCSA work different ways with DNS Reverse Lookup
Check your DNS entries and see if all have proper Reverse Lookup. Maybe this could help.
But could be different issues, check here: https://kb.vmware.com/kb/2113115
Could be also if you use different Identity Sources or even password with special characters.
Check this ones also: https://kb.vmware.com/kb/2060746
Also dont forget to read all vCenter upgrade documentation: vSphere 6.0 Documentation Center
Hope this could help.
Thanks for the guidance. I resolved this issue by re-entering the "firstname.lastname@example.org" password in the Migration assistant helper on the windows vCenter server, and running the upgrade again. For anyone reading, remember to double check your passwords.