WP Ultimo uses the file /wp-content/sunrise.php to bootstrap the functionality required for domain mapping. Though this file was likely transferred via your chosen migration method, the integration also requires a constant to be set in wp-config.php. If you’re using the default config provided by MightyBox, this constant will not be set.
To remedy this, open wp-config.php and add the line:
somewhere above the line:
/* That's all, stop editing! Happy publishing. */
Clear your page and object caches and your custom WP Ultimo-mapped domains should function as expected.