The username is limited to 30 characters which is not enough for our use-case. I assume we're an edge case but not the only ones.
Using the PAM Backend we need to login using fully qualified usernames (e.g. lfrancke@AD.......LOCAL) which are (in our case) 31 characters long
Someone else reported the problem here as well: https://community.cloudera.com/t5/Web-UI-Hue-Beeswax/HUE-username-lenght-limit-30-character/td-p/39621