Dear All,
So here is the final verdict on Sierra and failing folder opening:
Espionage uses encrypted sparse bundle disk images which are a data format provided by Apple's OS X. It is a clever and flexible approach of storing data safely. This is why we went that route and it worked fine for many years.
Apple also provides a command line utility to manage and manipulate those disk images, this utility is called hdiutil.
After troubleshooting the problem, it turned out that if we specify to the hdiutil a mountpoint folder path which is located on an external volume, then the hdiutil returns a "no mountable filesystem found" error in spite of all data being in good condition. It is sufficient to specify a mountpoint folder which is on the system disk and all starts to work fine.
The problem being in Apple's utility, we cannot do much more then report it to Apple. At which priority will Apple address this problem, we have no clue.
Till then there are two workarounds:
1. move the mountpoint to the system disk, as we suggested earlier
2. do the 1. but create a so called symlink on the external disk where the original mountpoint folder was located. This symlink is very similar to Alias of a folder in Finder, it is a folder which "points" to another folder. In this way you could access the content of the protected folder in a same way as before, although there would be two mountpoint folders, one real, on your system disk and one which would look as real, but would in fact be a link to the one on the internal disk. If you want me to send instructions how to create the symlink, let me know.
Thanks for your patience in this matter,
Rgds
Zsolt