[johnnym]

Hey, looks like these double-height memory modules really exist for Personal Workstations:

https://www.ebay.de/itm/133484664080
https://www.ebay.de/itm/225365908048

To my eyes these look very much like buffered ECC DIMMs (except maybe for the square chips in addition to the buffers). No idea, why the buffered ECC DIMMs from PCs won't work correctly in a PWS. Maybe I should retest with some PC-100 modules.

EDIT:
One those above has part number 54-25158-DA, which seems to be a 256 MiB (32MBx72) 83MHz module according to https://www.mkd-gmbh.de/gebraucht.php. There is also a PC100 ECC registered module listed manufactured by Camintonn.

Other thought, maybe buffered/registered DIMMs require a specific jumper/dip switch setting?

E.g. a Sun Blade 150 would only run with buffered ECC (PC) DIMMs with a specific jumper set on the system board but otherwise only worked with unbuffered ECC DIMMs. Strangely the jumper is labelled wrongly so implied the opposite IIRC.

@MrWeedster:
Any luck with other buffered ECC DIMMs?