Closing my first report. When I deleted the user from the group "sudo", everything worked back as normal.
However, IMO the user must additionally be in /et/suders to get the described behaviour working. What is sure: Either KDE or LXDE gave me the opportunity (by using the root password), to remember the passsword and let users execute applications (like synaptic) s root. I am glad, to find the reason at h eend. There is nothing more annoying, than not to kow, what is going on. Thank you all for your help and your thoughts abut this problem! I love the community! Best regards Hans
