I am having the same problem as well and still trying to narrow down what cause the problem.

Detail description of my problem:
Like Dkp1988 wrote, the system will sometime stuck at boot with a "_" (it does not blink on mine) on the screen without showing up any post message.
QCode shows "AE".
- Power-off and power-on the system (using the power button on the case) does NOT always get rid of it.
- Using the Restart button does NOT get rid of it at all.
- Power off, clear CMOS, and Power-on DOES always get rid of it.

I begin experience this problem after:
- Flashed bios to 2001
- Bought a Logitech G910 Orion Spark
- Did a clean installation of Windows 10 Pro, and installed few updated versions for the component drivers.
I am thinking the culprit is bios 2001, because the other two does not make any sense to have bios not post at all.

I have tried the following steps when it happens.
- Un-plug all USB devices and restart (using restart button) to see if system will post. NO GO.
- Re-flash bios 2001, no problems for two days, but just had one this morning. NO GO.

Steps I am going to try if it does happen more often
- Un-plug USB internal connectors on the motherboard.
- Flash back to 1901, and see if the problem would still exist.

Dkp1988, maybe it would help if you can try to recall what you did just before you start to have this problem.
And what steps you have tried.
This might help us narrow down the cause more efficiently.

phaalan wrote:

I begin experience this problem after:
- Bought a Logitech G910 Orion Spark

Not saying it's 100% that the keyboard is the culprit but like I said before. The AE QCODE normally is caused by an USB device. For example, I have a generic gamepad which will always cause the system to halt with the blinking _ cursor and the AE QCODE. Other users have reported getting this same problem from mice, headphones, joysticks and such.

The easiest way to test if an USB device is causing the problem is to disconnect ALL of them whether it be front panel USB ports, card readers, mice, keyboard etc... Now let's say you find the culprit. What can you do about it? The first step is to go to the manufacturer's website and look for a firmware update for your device. If none is available and the manufacturer doesn't plan to offer one in the near future, you can disable xHCI as a workaround but you will lose USB 3.0 functionality. Users have reported plugging the USB device into a USB hub as a workaround as well. Or you can just replace the device with a compatible one.

Thanks Chino for lending a hand to help.

So I did tried to unplug all the USB devices when the problem occurred yesterday. And I restarted the system using the restart button in the front IO.
But it doesn't get rid of the problem.
The reason I used restart button is because sometime doing the power-off and power-on with the front IO power button makes the system functional again without doing anything else.
So it won't be able to help in narrowing down the culprit.

Right now, I am on bios 1902, and haven't had any AE or "_" problem all day so far. But who knows.
I'll keep monitoring it.

So Chino, let's say the problem only happens in bios 2001.
Does that mean the new bios has a bug or it simply maybe a USB hardware/controller & bios conflict?
I don't see myself giving up on the USB 3.0 functionality though.
If that's the case, I would have to weigh in on the benefit of staying on bios 2001 vs losing USB 3.0 functionality.

I have this problem when i have 3/4 devices plug into to my usb on my motherboard... Very rarely tho

I decided to flash bios 1 to version 1901 and reflash bios 2 to version 1901 then to version 2001.
I will run the system under each bios for couple days with default settings, and see if the problem exists.
I'll update with whatever I found.

By the way, Dkp1988, which bios version you are running on?