I'm not the original poster and haven't done this, but if you have no other luck you could try changing the domain name on the ReportServer.dbo.Users table UserName column.

Other places the old domain name is stored are the sys.server_principals and per-database sys.database_principals tables. You can rename these with Alter Login / Alter User With Name = and the new full name on the new domain.

Maybe the SID history works but when it resolves the internal names to the new names it messes up at one of these points.