Username does not pass the minimum requirements

What does that mean!?! What am I doing wrong?
Please write useful error messages!

I've that error on the initial page/form to create the admin account after having installed this plugin.

No field (especially not the `username`) is marked as wrong/error.

I tried first with something like `abc.def` in the username, then with `abc-def`. Both produce that error. Then finally `abcdefghij` worked.

Funny thing, creating a new user via the admin panel works fine with `abc.def`. The validation process/rules are apparently not centralized.