So this is the issue at hand:
I own a lot of additional "libraries" for SampleTank 4. Some of them stopped working in Ableton for no reason that I can understand.
They still work in Pro Tools and in the Standalone version. Also the VST 2 Pluginversion works, it is only the VST 3-Version that is affected. That's still unacceptable, because I have a lot of productions where I had the VST 3 Version of SampleTank 4 loaded and thus the according tracks simply don't play now.
Reinstalling didn't do the trick. The instrument-folders are obviously working (VST 2 and AAX work). There's something up with the VST 3.
Examples of libraries that don't work are:
Electric Grand C70P
Electric Piano Bass
Fractured Piano
Malletension
Malletopia
Shipwreck Piano
Stage 73 Mark V
The Resonator
Wurly Model 120
SampleTron 2
Syntronik 2 CS
This does NOT affect all libraries. Examples that work are: Alternate Keys, Clavitube, Philharmonic 2, Syntronik (1), the "Drum Series" and a lot more.
I THINK it stopped working around the time I installed Syntronik 2 - that's speculation. It seems to only affect more recent libraries from what I can tell. I need this fixed, so if anyone has any insight, or if IK Multimedia is aware of this issue: Please tell me!