Hi James,
thanks for your effort, points will be rewarded!
We found that the log partitions between source and target system were different: The source system has 4 log volumes with 2 GB each in 1 partition. The target system also has 4 log volumes with 2 GB each but only log volume #1 (DISKL001) belonging to log partition #1 (other log volumes were sort of unattached). We fixed this by putting log volumes #2 to #4 into log partition #1 via param_directput.
After a new "db_activate recover <medium> data autoignore" the database finally came up!