iMac fails to boot - graphics issue?

Solution 1:

This panic signature:

panic(cpu 0 caller 0xffffff7facc1f244): "virtual bool IOAccelLegacyDisplayMachine::display_mode_did_change(uint32_t): AMDRadeonAccelerator driver returns false" @IOAccelLegacyDisplayMachine.cpp:267

along with this fact:

It does boot fine in safe mode...

strongly implicates the graphics stack in the failure sequence, since the panic signature is inside the accelerated graphics driver and this driver is disabled in Safe Boot.

The fact that your standard troubleshooting practices, including a full OS reinstall, failed to remediate the issue gives strong support to a hardware failure.

How to test this: I expect that if you run Apple Diagnostics (hold D during boot) you will see an error.

GPU HW failure does happen to a steady percentage of machines over time and there's nothing that you can do to resolve it yourself on an integrated machine such as the iMac.

--> Conclusion: You should bring your machine to Apple for hardware service.