I finally found some time to open my oscilloscope, connect to the UART (connector 901) with a USB-TTL adapter and get access to the linux command line. After getting a full backup, I tried to manually perform a firmware update, using the latest unencrypted firmware for Hantek DSO5102P (which supposedly shares the same h/w with my ATTEN). The update completed successfully, so when I restarted the scope I could see the new firmware version in the system information window (see attached).
However, when testing the new firmware I noticed the following problems:
* Some scope keys did not respond correctly. Ex. when I pressed the "Measure" button the "CH1" menu opened instead.
* Self calibration failed with an error code 0x802
For the above reasons I rolledback to the original firmware from ATTEN (again with the linux command line). My best guess is that my scope model is very close to the corresponding Hantek model, but with minor differences. If someone has an idea how to correct the errors listed above I can retry to use the Hantek firmware.
BTW, I managed to extract the gpg passphrase for my scope model, which is "ads1000_40k". This was hard-copied in the dso.exe file, so I used a hex editor to extract it. I have kept full backup of the NAND memory and the complete filesystem, so anyone who wants anything can PM me.