OK one day later, I found the problem.
I checked wvdial. And there was no modem defined. wvdial told me that there was also no /dev/modem file.
So I looked up the ttyS* configuration in /etc/sysconfig/hwconf. My modem was was on ttyS0. Then I created a link from /dev/ttyS0 to /dev/modem, which was missing.
And now it works again.
There's yet another problem. Every system restart I have to do this procedure to create a new link. Is there any possibility to create a permanent link? What's matter with this deletion of the link? Is there a program witch deletes the linkfile?