@gerald.stachl
Seems. that the problem is solved:
I found another thread: https://forums.fogproject.org/topic/6129/multicasting-stuck-on-starting-to-restore-image/36 where Tom gave the advice to remove the directory /opt/fog and start the installer again.
I did that, and after that i saw, that all the logfiles are created successfully. Then i cleared the old tasks
mysql -u root
use fog;
truncate multicastSessions;
truncate multicastSessionsAssoc;
truncate tasks;
after that i rebooted the server. Now newly created Multicasttasks work! 
Thanks for your help!
My Multicast Tasks are starting now, but (i tried it 3 times with different images (all of them working with unicast)) they are not finishing => shall i open a new thread and mark this one as solved?
EDIT: Did other tests with multicast. At first i tried a small image (Zorin Linux - 8GB) it worked. After that i tried one of our Windows 10 Images again (~70GB) and it worked also.
So i mark this case with SOLVED!