"You must be a domain administrator in the target domain. SID migration will be disabled."

After 15 days of frustration and constant nightmares, with no help from Microsoft forums, white papers, articles, etc.,
I finally figured out how to get rid of it, here is how you can solve it:

1.- Log on to a SOURCE domain server as SOURCE domain administrator .
2.- Edit Group Policy -GPEDIT.MSC-
3.- Navigate through Local Computer Policy->Windows Settings->Local Policies->User Right Assignment
4.- Grant TARGET domain administrator the "Allow log on locally" permission in the form of "targetdomain\administrator"
5.- Force policy to apply with the command GPUPDATE /Force"
6.- Log off
7.- Log in again but as a "targetdomain\administrator"
8.- Go to Start->run and type MIGRATOR.MSC
9.- Right click on Active Directory Migration Tool
10.- Select User Account Migration Wizard and follow instuctions
11.- Once you get to the "Account Transition Options" screen be sure to select "Enable target accounts" and tick "Migrate user SIDs to target domain" ..and voila, no more error message

Hope this help, in my case after 2 weeks of trying to find out ho to solve it, no web site or blog was helpful. Fell free to leave your comments.

Regards


Jesus Robles, Consultoria Avanzada en Sistemas
MCSA, MCP