thanks for the input, guys.
right now, what i did:
kicked EVERYTHING off the computer (of course using Mac install CD), 'erased disk' (4 of them: i have two seagate barracuda's, one of them is divided into 4 partitions, the other is used only for data, like audio. this one's divided into three partitions). then i used 'drive setup' just to be sure.
ok, so now i must have a clean HD, or could there be some other thing lurking in the shadow, like a virus? (i don't have any anti virus stuff, must be my mac arrogance

any suggestions or advice?)
yesterday, i installed a fresh mac os9.1 on one of the partitions, did all the obvious changes and installed internet, printer,... just to keep in touch with Z...
then i did the same on the second partition, where my DAW is supposed to be. SFP installed normally imo. then i tried to add cubase 5.1r1 from CD. it goes no further than '731 files to go'. if i copy the CD to HD, i'm able to install cubase

but i still get no answer when i knock it's door. being rather frustrated, i threw off everything again (i must say, only the drive containing OS).
i tried everything: installing before tweaks, after tweaks, in between,...
this morning i spent some hours browsing for an answer. but nothing really valuable came up.
i just made a fresh OS install, and wanted to download SFP again, and maybe cubase if it's available to eliminate possible CD errors. and of course i popped in here to check the Z. so that's where i am right now: nowhere. i realized i forgot to assign, lets say 200Mb to cubase. maybe this could solve my problem. i doubt it, because when i just started out, i didn't know ANY of the optimalisation tricks, and everything worked back then... oh, yeah, the cset.ini file: i did in fact change it to a copy i made in the past, but there was no real need for it, because SFP is working fine.
ah well, when all this is over, i'll at least know a bit more how computers work... i hope
system:
B/W G3 450Mhz, 512Mb RAM
2 seagate barracuda IV HD's (40, 80GB)
Mac OS9.1 (9.2.2 upgrade is said to be useless on a G3)
pulsarII/pro pack/SFP, Cubase 5.1.r1/r2 (installing the patch over the original brings no solution), OMS 2.3.8 (has always worked for me. i have 2.3.7, used it with the same success as 2.3.
my atari is justed gathering dust on a shelf, less than 5 feet away... mmm, just like my BIG soundcraft mixer. the nordrack2 is always close at hand, so are my outboard FX processors... too bad i sold my drummachines