R340 does not power on, system board diagnostic LEDs blinking

Hello, I have a problem with Poweredge R340 and need information regarding its motherboard diagnostic LEDs reports.

Preamble: I've bought a used server from half a world away, and shipping it back will cost more than I've paid for the server. So I could not return the server to the seller because it's not feasible, and I could not contact Dell support directly because there is no more warranty with this server. 

The server came in a working condition - I was able to power it on, enter BIOS, change some settings like the iDRAC network configuration, reboot the server, and so on. Then I've rebooted to a LiveCD to do some other checks like HDD S.M.A.R.T report, and after some time server powered off itself and never powered on since then. The server was working for a few hours total; I did not even run any CPU/memory intensive programs - the server died when I was reading through the `dmidecode | less`.

So currently the server does not power on, and when I press the power button the fans start blowing, the motherboard LEDs blink differently, and after 2-3 seconds server turns off itself.

This is what System Board LED Indicators look like:

So the power on sequence is like this:
0) Server is turned off. LEDs 2 and 4 are on, LEDs 6 and 7 are blinking. I press the power button, then:
1) LEDs 3 and 4 are on
2) LED 5 is on
2.5) (some LEDs turn on and turn off but too fast so I could not notice which ones exactly)
3) LEDs 1, 3, 4, 5 are on
4) Server turns off itself. LEDs 2 and 4 are on, LEDs 6 and 7 are blinking.

I was not able to find any information about LED indicators for a fresh servers with 7 LEDs, Dell website offers only old information about 10-11th generation servers with 4 diagnostic LEDs.

However I've found one document about a different server but with 7 LEDs too (VxRail P580N):
https://docs.delltechnologies.com/bundle/RAIL_OM_P580N/page/GUID-DA28719E-E1BC-4E32-B172-C218214EB572.html

This document tells that it could be a CPU voltages problem:

LED7 LED6 LED5 LED4 LED3 LED2 LED1
B...........B.......○......●.......○.......●......○......CPU1 VCCIO Failure

I think the meaning of these lights for R340 could be different because this server has only one CPU instead of two as does VxRail P580N.
Also concerning the voltages - I did not enable any overclocking or high performance settings in the BIOS, and reset BIOS to defaults using the NVRAM_CLR jumper.

The iDRAC does work well, but there are no any errors (except the "Chassis is Open" of course) in the iDRAC System Event Log.
Another strange thing is that the server does not respond to "Power ON" action from iDRAC, only to physical power button press.

What I've tried:
- clearing NVRAM with a jumper on the motherboard
- updating iDRAC, resetting iDRAC to default settings
- removing all unnecessary components such as PERC or TPM
- removing memory modules except single one in DIMM A1 slot; and replacing it with a definitely working one
- removing ALL memory modules (but the motherboard diagnostic LEDs report was the same - LED 2 and 4 on, LED 6 and 7 blinking)
- changing the CPU to a definitely working one
- removing the CPU (the motherboard diagnostic LEDs report changed to: LED 1 and 7 blinking)
- replacing PSUs with definitely working ones

So most likely it's not a problem with dead CPU or RAM or PSU.

What I suspect:
- power distribution board is faulty *1
- motherboard is faulty *2

*1 - I suspect the PDB may be dead because when server is powering up the strange buzzing sound comes from the drive backplane for a second, just before the server turns off itself. I do not remember hearing that sound when the server was in a working condition. Also note that the server does not respond to "Power ON" action from iDRAC.
So if the PDB is dead then it could really be a CPU voltage issue.

*2 - I just hope this is not the case... But the LED indicators are the same even if I remove all memory modules and try to boot the server. So it could also be a memory controller issue.

Please tell what does this LEDs status exactly mean (LEDs 2 & 4 on, LEDs 6 & 7 blinking) for PowerEdge R340, and suggest what else I should check.

Thanks in advance