Hi!
I did this on two separate X/Ubermentu websites, and had to restore backups on both.
I have a CSS classes screen option turned on, and I have added a css class to one menu item. The moment I did that, the whole navigation broke on both sites. Many items were lost, and the Ubermenu itself lost images and formatting.
While here, let me also report that LastPass Browser addon has conflict with WP menu system. This is probably not an X/Pro thing. However, I disable that addon each time when I work with menus, and I didn’t have it active this time either.
Thanks!