Hello @perdrix
I tested it on my side, the usb bootloader can work well with win11.
Please test directly program the bootloader.hex file, not build by yourself, and before program, full erase the chip. Then power-up again.
There should be a disk under my computer:

In Device manager, there is USB Mass Storage Device:

If still can not work, please check jumpers on your board refer to <AN10866>.


BR
Alice