When I try to clear the textfields there to make them blank and click on "OK" the changes aren't performed. So the old content is kept in the database. But when I clear only the fields in one of the panels ("User Profiles", "Home Directory"), the action is performed in the right mode.
The issue was more general then that - no string could be removed to "", only replaced with another value.
I've fixed this in -r 24059.
*** Bug 4823 has been marked as a duplicate of this bug. ***