After typing in a few letters in the To field, it suggests some users. If you click on one of those names with mouse, it populates the field as undefined.
BUT - if you arrow down with the keyboard and hit enter on the name you want, it populates the field with the desired name just fine.
Ok, then it is a problem with your site (maybe a side effect with another component). Check the Javascript error console, maybe it gives a hint what is wrong.
Maybe FAQ 10.5.1?
uddeIM & uddePF Development
CB Language Workgroup
CB 3rd Party Developer