Josh, thanks for your hints and sorry for my late answer. In fact calling kpersonalizer was helpful. While I had the strange fonts for the first two or three screens after one "Next" I've got nicely readable fonts and thus the problem was solved.
Now I leave it to you to find out which setting would care for the font as a reasonable default if somebody works like me and does not call kpersonalizer but instead run KDE appllications from Gnome or any other WM. ;-) Please tell me whether you need any further hints to debug the problem. Kind regards and thanks for your help Andreas. PS: Feel free to downgrade severity from important to normal because I think if this problem is documented in this bug report the severity can be lowered.