Computer does not properly boot or crashes without monitor

[burnum]

I want to use my HP ProDesk 400 G5 in headless mode, i.e. without monitor. OS is Ubuntu 20.04 LTS.

When a monitor is plugged in (DisplayPort), the computer boots correctly and everything works fine.

Without monitor, the computer boots and usually crahes very few seconds after the (automatic) user login. Then, it reboots again and again. I plugged in a USB-Device and interposed an electricity meter so I can see that the USB power turns off for a fraction of a second at that time. Futhermore, I added a sequence in the ".profile" to write date and time to a logfile to analyze the situation. So it seems that sometimes the crash occurs already before the user login. 

Strangely enough, sometimes even without monitor,  everything works properly. And then, out of a sudden after some minutes and sometimes hours, the computer crashes again, and then reboots again and again.

What I tried so far:

- installed a different Ubuntu version (21.04) without success

- reset BIOS to factory default without success

- plug in display port cable without monitor does not solve the problem

However, it seems to work when the cable is plugged in to a monitor even though the monitor is off and has no power supply. It also seems to work to unplug the monitor after the boot process. But I have not tested both cases for the long run so far. Unfortunately, this is also not a permanent option for my application.

Maybe also interesting:

- I have several of the HP ProDesk 400 G5 in use. The problem occurs not for all but for some of them.

- I added a "beep" to my ".profile". I can hear the "beep" when the monitor is plugged in, but not when it is not, even in the cases when everything works and started correctly.

[burnum · Accepted solution]

OK, I still don't know the exact reason, but found an easy solution.

For little money, beginning at about 5-10 EUR (or USD), you can buy so called Display-Emulators. E.g. Display-Emulator on Amazon. They let the compter think there is a monitor. With this emulator, the headless mode works fine.

Still, I would be interested in the cause of the effect.