I was (probably) on the last boss in act III. After a cutscene the boss didn't move and was kinda invunerable. Since nothing I tried changed it, I quit the game (menu not desctop) and wanted just to restart from the last checkpoint. Unfortunatly the game then decided that I am on act 1 with 0 progress. I lost all my story progress and can only start from act 1 (nothing selectable for act III), so I guess this is a double bug report, the endboss was broken and leaving to main menu while boss fight deleted all my story progress.