Found it:
From:
https://www.yourwindowsguide.com/2021/08/troubleshoot-windows-11-upgrade-failures.html
C:\$Windows.~BT\Sources\Panther: File named Setupact.log records actions in the downlevel and SafeOS phase. As the log can be very large, setup also creates a file named “Setuperr.log” that only has information about the errors encountered by the Setup to narrow the source of the problem.
And from the same article:
Error Code Troubleshooting
C1900101-20017 A driver has caused an illegal operation. Make sure all unnecessary devices are disconnected except the keyboard and mouse.Remove any 3rd party anti-virus.Make sure BIOS firmware is updated on the motherboard.
Now to figure out what driver is causing it.