Hmmm... the problem ist that it is only possible to set a passwort in the categroy admin when i create a new category. If i want to set a password for an existing category the pw ist not written in the database.
On the other hand, if i have set an initial pw i can delete it by simply enter the old one without a new one in the category admin. But after this it is again not possible to set a pw again.
There is no error message.... Strange....