In a freshly set up Skype for Business hybrid environment we got this error after trying to move the first user from onprem to online:
HostedMigration 510 error : the user could not be moved because the tenant has not been enabled for shared SIP address spaces
After verifying that both the tenant and onprem environment is actually set up for shared address spaces with
the issue is still there.
It turned out that the user were homed in a child domain and the FE pool in the root domain. The problem was solved by running the Move-CsUser Cmdlet with the -DomainController parameter as well, like this:
Move-CsUser -Identity firstname.lastname@example.org -Target sipfed.online.lync.com -Credential $cred -HostedMigrationOverrideUrl "https://adminXX.online.lync.com/HostedMigration/hostedmigrationService.svc" -DomainController dc.contoso.com
Remember that your HostedMigrationOverrideUrl will probably be different, just log in to the SFBO admin portal to get yours.