Ultimenu 2.x is a major rewrite to update for Drupal 8.6+, and add new features.
It may not be compatible with 1.x.
The following applies if updating from 1.x. If not updating, simply ignore.
Ultimenu 2.x added few more services, so it may break the site temporarily mostly due to the introduction of new services till you run the update below. If you do drush, this is no issue.