That ferry is a special place but I think it goes both ways so to say.

It was probably more of a thing before Sweden joined EU though. Alcohol is no longer half the price when you go abroad.
On the original topic I thought I'd just give an update for future reference in case someone else should experience the same problem. Here's what I did:
* Reseated the dimms. -> That didn't do anything good in my case but it's probably the first thing to try still.
* Removed all memory sticks but the last two in bank 1. -> Errors went away and I could install the system.
* Cleaned, sorted and reinstalled all dimms in correct order. Rubbed the dimm contact pads with an eraser and then wiped them off with some alcohol. For cleaning inside the slots I cut a strip from a toilet paper roll and dipped in alcohol.
Found some useful info on how to pair and sort the dimms on this link:
https://wiki.preterhuman.net/SGI_O2#Memory
Interestingly some of my dimms that are marked 128Mb are acutally only 64Mb each. The color coding is correct though so I assume the capacity sometimes refers to the single stick and sometimes it's for a bank of two. Just something to look out for when sorting the banks.