So is this your machine: http://ps-2.kev009.com/pcpartnerinfo/ctstips/594a.htm ??
error - 000165XX or 000169XX means Set Configuration/Features. So the first 00016500 means you need to run the Config tool SC.exe.
according to this list of some codes: http://ps-2.kev009.com/pcpartnerinfo/ctstips/49fe.htm.
Also this from the PS/2 Maint. manual: http://ohlandl.ipv7.net/books/ps2-hmm.pdf
"Symptom: 00016000, 000164000, 0016500, 00016700, 00016900 (If setting configuration,date, and time does not solve the problem, see “Devices List” on page 23)
Action: 1.Set Configuration,Features 2. System Board"

Do you have the Reference Disk ? see here: https://www.ardent-tool.com/8556/8556_Planar.html
Also it appears to use a Dallas DS1285 for CMOS which uses an external battery, you should replace the battery before running SC.exe or else the changes will be lost at power down.

added: the second error:
"0037XXXX (This is usually caused by the SCSI controller built into the system board)
1.System Board, 2. Hard Disk Drive, 3. Hard Disk Cable
the third error:
000181XX (The computer requires a network adapter for remote IPL or a hard disk drive ID of 6 LUN 0, for IML; neither of these were detected)
1.Run Automatic Configuration, 2. Network Adapter, 3. Hard Disk Drive, 4. System Board"

That is all I got !

Yes, that is the exact model!

Thanks for the tips!

I've tried the reference disk, and actually it is the only disk the machine is willing to work with. Others, like dos boot disk/install disks do not work.

I have to run the config tool sc.exe.

I have tried this https://amigastore.eu/en/600-scsi-to-micro-sd-adapter.html with the machine just to check out if there is any way just to try is everything ok with the hardware. Diagnostic tests all worked without any problems. I guess 8556 is really picky with drivers as it seems I need to get every possible driver just to install dos and everything else, if I'm correct?

I have to take pictures of the devices the machine was installed with (micro channel bus cards, such as display adapters etc.) if anyone might know where I could possibly find the missing drivers.

This particular IBM model has been quite interesting as an on/off project, because I have randomly tried out to make it work, but I hope that I will get the machine working as it otherwise in perfect shape!

Are you trying to boot IBM DOS or MS DOS ? I would try IBM DOS 5 first. Then I would try IBM DOS 6.x in case MS DOS does not like the machine type .
I would then try: disconnect the SCSI HD cable and re-run SC.EXE and set no HD if you still cannot get IBM DOS to boot. It is possible the old SCSI HD is dead or malfunctioning...