Buffered ECC 3.3v 168pin DIMM. 32mb, 64mb, 128mb or 256mb. From one of the manuals, it might be able to boot with just one DIMM.

https://www.ibm.com/support/pages/features-pc-server-330-0
According to this page 128 MB is the max size supported, but support 256MB sticks might be an undoccumented feature.
256 MB do exist but are hard to find, while smaller ones can be easily found in older (Compaq-) servers.
The sticks you are looking for have 9 or 18 chips of the same size and two small ones for the buffering.
Here is an example:
https://www.electromyne.de/RAMs---Memory-Vint … 228469-001.html

Thanks for the replies and the help!
It’s really unclear in the hardware manual if it should be able to boot on one stick but I kinda hope it can’t, because it seems a bit unlikely that both the original modules should be toast. But if it’s not the ram I can’t really figure out what could be wrong, both CPUs are ok during POST so the system board should be OK then I guess. I’m gonna try ordering 4 new sticks to populate all slots and see if that works.

At least for similar Compaq and hp PPro boxes the memory modules does not need to be installed in pairs.
Did you have a closer look at the memory sockets? These caused trouble in some of my machines.
To clean them you can use a piece of cardboard the thickness of the memory PCBs, soacked with isopropylic alcohol.
Some manufacturers warned about mixing tin plated modules with gold plated sockets and vice versa,
but i never experienced problems caused by this combinations in personal.

I bought a couple of new ram modules and with those installed it passes the ram test during post. But unfortunately it still doesn’t clear post, it gets stuck further on with no signal to the monitor and no beep code. I put a simple post code analyzer card in it and eventually managed to get it to display codes, but none of the codes displayed seem to correspond to anything I’ve managed to look up online, so I’m guessing it’s some proprietary stuff with no openly available documentation. Would be so cool to get this old beast up and running so if anyone has some knowledge on troubleshooting these machines, any help would be awesomely appreciated!