Good Morning Jens,
OK, in this case it might be that you are hitting the "external volume mountpoint folder on Sierra" problem.
Please create a new empty folder in your Documents folder on the boot disk, name it anyway you want.
Now unlock Espionage, click onto small "i" near the folder name in the espionage folder list, and using the mountpoint drop down menu, point Espionage to this newly created folder.
Try to unlock the folder now, if it works, you will see the content of the disk image through that new folder.
Let me know if this worked,
Thanks
Zsolt