Empty error message when creating new user with no unique name

Reproduction-path:

• Login (admin) into CMS (for example Hippo GoGreen Demo: https://cms.demo.onehippo.com/)
• Create new user (via Admin perspective -> users -> create user)
• Fill field "Username" with already existing username (for example "admin04")
• Fill fields "Password" and "Confirm password" with valid and equal passwords
• Click button "Create"

The new user is not created, but a yellow bar without error message is shown (see attachment).