@george1421 We have 3 switches between the computers and the FOG-server.
The problem is resolved!
Our Solution:
The WebGUI from the switch showed the options in the IGMP Snooping as enabled, in the config-file some of the needed options were disabled. After enabling them in the file and uploading that we tried another mutlicast task.
The timeouts still are in the multicast.log.udpcast.x, but the task completes successfully.