You know what, in the end it seemed so messy that I just spent one day reinstalling OS9 and all the software from scratch.
This time, I wrote a precise log of what software I installed, an in what order. I also gathered all the serial numbers to one handy place and documented any possible annoyances, to make reinstalling easier in the future if I need to do it... I also labeled the extensions and things before starting to install anything so I could log what each installation adds, and where.
This time, I installed Photoshop right after Toast and the initial setup, and it didn't mess up the computer. So I went ahead and installed all the other software too. I haven't seen any oddities so far, it's working fine.
The installing order wasn't the only difference here: When Photoshop messed up my system earlier, I was using a Photoshop 4 installation, and then applying a Photoshop 6 upgrade after that. I thought it was a messy way to get Photoshop 6, but at the time I either find any other alternatives, or I was already too overwhelmed by the long lists of different download options of the same thing, that I picked the wrong method. Now I installed Photoshop 6 full version directly, and got no issues. If I understood correctly, this machine could run Photoshop 7 too, but according to comments, it oddly dropped gif support and (I guess) probably isn't significantly different to Photoshop 6 in other ways, so I chose 6.
Yes, it is possible to have multiple system folders and yes, that would confuse the Mac and can cause all kind of problems. So check if there's more than one.
That sounds so strange! But yeah, I checked before reinstalling the OS and there was just one System folder all along.
I too think it's an issue of an extension being overwritten with an older version. A good utility to keep around is Extensions Overload. It's a tool that shows you what Extensions (and Control Panels etc.) are installed on your system, what they do and lets you deactivate them. That's a good way to test and find "the culprit" causing trouble.
That sounds exactly like what the OS's own Extensions Manager already does. What's the difference?
As a general rule, it's a good idea to keep a copy of your freshly installed and once setup System Folder around. Unlike modern OS, you can just copy the folder onto e.g. an external HDD and once you need it again, copy it back. Not that I much needed to do that in 30 years of using Mac OS, but it sure would help in your case.
Thanks, will do. It's funny because that backup doesn't even take long to do to a USB stick. But I chuckle at the fact that the computer calls it a "cartridge" !
If I were to copy a System Folder backup back, would I have to do that by first booting from the OS9 installation CD, or can I copy it while I'm "logged in" normally?
probably because textencoding converter is missing from your systemfolder and the app needs it.
i just wonder how it can get lost by a PS 4 installation.
otoh, adobe also managed to cause data loss with some PS update for windows in 2020. maybe they are simply idiots.
Yeah, that's the thing... I ask because I don't know what textencoding converter is, and whether it's something that exists in the OS or is an extra component installed by some third-party software.
but after a crash or PRAM error then you might end up unnoticed in an other one than you think... and then suddenly your prefs are gone and stuff. 
Oh that sounds awful for sure.
I think that the Hard Drive or the memory are rotten. That corruption on the disk happened to me only on G3 B&W rev.1 with bad memory + bad Drive + faulty battery
I sure hope it isn't that, but taken the computer's age, I'm well aware of the fact that it just might be...
So far, the computer doesn't seem to crash much, and certainly not randomly. It's very responsive and runs smoothly.
However, I can at least confirm that there definitely is an issue with the battery: First of all, the machine won't turn on at all unless plugged in. Secondly, the battery isn't showing any charge when it's plugged in. If I unplug and plug in again and then boot, the clock resets to 1.1.1904. From the Date & Time panel, I've chosen to update the clock from the European Apple server whenever there's a mismatch, but in reality the machine won't update the clock spontaneously, I always have to go to the control panel and manually assign it to fetch that time now. It seems to only lose that time if the computer comes unplugged, but it will keep the time if I merely restart the computer while still plugged in. I'm too much of a novice to tell whether this is a dead battery, a dead capacitor, or both. If it's a dead battery and all it causes is the annoyance with the clock (are there any remedies?) as well as the fact that I can't use the laptop on the go, I can live with that. But if there's something wrong with the capacitor that can cause something to leak and cause damage, then I believe I should be doing research on what to do about it.
While the hard disk noise is surprisingly not the most annoying one out there, I do love it when it stops spinning. After a little bit of research, I've understood that the clamshell is fiendishly hard to open which worries me, but for the sake of the old age of that hard disk, it would probably be a good idea to start considering a new hard disk.
It's got 320MB of RAM, so I guess that can still be upgraded too (and wouldn't hurt in case the RAM is rotting?)
Where, how, and at what cost are of course going to be the big questions...
And, the audio jack seems to be partially faulty too because left channel is only playing very quietly, making it impossible to use headphones with this thing.
(If this is too off-topic by now, I can also start a completely new thread on "what hardware could I renew, and should I")
