as from my own experience, the downgrade process went up to 100% without warnings or any errors.
The 1% issue seem to be really weird. Indeed, log is required, @Elysium. What you could do is:
1. Run the lib
2. Attach USB with firmware downgrade file (unzipped of-course!) with T-GAS6DEUC directory in root of USB
3. Navigate via menu and start "upgrade" trough USB
4. When you are asked to reboot, connect to tv via ftp and get /dtv/FWdowngrade.log
5. Click on reboot (or what you supposed to press on to continue downgrade)...
6. Attach log file here
