Finally, I made a little step in the right direction:
With "cyfwstorprog", I programmed the image successfully and started to search for differences. The only one was that partition BOOT1 was set to CY_U3P_SIB_LUN_DATA instead of CY_U3P_SIB_LUN_BOOT.
As partitions BOOT1 and BOOT2 are not in the USER partition part, I have no access when partitioning the device, so I cannot change them using CyU3PSibPartitionStorage.
So my question: How can I modify the partition type of the boot partitions?
Found out that by myself, in meantime: It's done by patching the Ext. CSD register 0xB3 (i.e. using "CyU3PSibSendSwitchCommand").