Eduard, could you try editing /usr/share/grub-gfxpayload-lists/blacklist/20_radeon_hd6800 on your system to add a second line:
v1002d6739.*
then run 'sudo update-grub-gfxpayload', remove the fglrx package, and confirm whether the system boots correctly without resorting to rescue mode?
Eduard, could you try editing /usr/share/ grub-gfxpayload -lists/ blacklist/ 20_radeon_ hd6800 on your system to add a second line:
v1002d6739.*
then run 'sudo update- grub-gfxpayload ', remove the fglrx package, and confirm whether the system boots correctly without resorting to rescue mode?