Code AA means everything is OK and working properly, this is what I get on the Xtreme after a successful boot when I'm in Windows, I looked in the manual and did not see any Code E6, are you sure it is not something else? At this point, assuming you did not make changes to hardware or flash a new BIOS, it could be anything such as a loose connection somewhere (maybe the GPU PCIe card is not sitting well for some reason and causing some intermittent issue) or the BIOS NVRAM could have corrupted itself causing some issues with your system configuration.
I would try to reset the BIOS using the clear CMOS button at the back and see if you still get issues, it could also be something as simple as your BIOS battery dying from old age and causing some problem (I'm not sure why it would prevent booting though, and you would probably lose your BIOS settings, therefore this may not be the problem), you can also try switching it out with a new one.
I would look into the GPU though and try re-seating as I had a similar issue that was caused by a defective AMD 5700 XT that would not reliably POST and hang at boot with PCIe 4.0 activated, once I RMA'd the card everything started working reliably with no other hardware changes (hopefully yours is fine and you don't have to replace anything).