There was a report in that topic that newer kernel-6.6.42 also works so you can try to use it:
You can try this:
But use this command to regenerate the GRUB config after editing /etc/default/grub:
sudo grub2-mkconfig -o /boot/grub2/grub.cfg
It seems to be a kernel regression but it’s not certain, maybe it’s some specific issue with using Xen and/or old Xorg version.
I guess you can try to boot in general Fedora OS with the same kernel version that is not working but without Xen and newer Xorg for a test.
If it works then I guess you can report this issue to the Qubes OS here:
And if it’s not working then report this issue to the kernel mailing list.