I installed Fedora 34 on Dell 5510 laptop, everything worked fine until I generated and enrolled MOK to use secure boot (used mokutil --import, then rebooted and enrolled in MOK manager). After enrolling MOK bios started to randomly block shim boot loader (more than 50% cases). I had to set secure boot to audit mode to prevent this.
EDIT: after testing a bit more I'm inclined to think that shim actually hangs rather than being blocked. And the only combination that allows it to proceed is secure boot enabled in audit mode. Enabled in deploy mode - hangs, disabled - hangs.