Stuck in the middle...

Hi all,
i'm trying to install BS on my 4.1 MacPro (flashed 5.1) - 32 gb ram - ssd HD - non metal gui
everything works fine with efi boot. After reboot I choose "Install Big Sur" and the Apple logo appears with the progress bar. In a few minutes the progress bar arrives more or less at 50% but no longer moves. I've tried to wait several hours but nothing changes...
Anyone can help me?