Logic Pro does not open the Sampletank 4 libraries

Oh, that's strange. Here it goes:

I'd suggest to try the following: 1) Restart the Mac; 2) Rescan plugins in Logic.

However, it's a bit difficult to discern from your post what exactly have changed since SampleTank 4 worked well. If your new Mac has a different version of macOS than your previous one, I'd say that might be the reason. However, as it works well with other DAW, it may be a bug in Logic or the Audio Unit itself. Besides, with other DAW you might be using a VST, not AU.

Could you, please, outline what troubleshooting advice did IK Multimedia customer support give you?

So what's changed in your setup is the macOS version, as your new Mac Mini has macOS Monterey now, correct? Could you tell what the other sequencer is (the one where SampleTank works well)? I mean, if both DAWs use the same format of SampleTank plugin (i.e. Audio Unit), then the issue is with Logic (and you could report that to Apple and / or IK Multimedia, so that they seek the solution for you). However, if the other sequencer uses VST format of SampleTank plugin, but Logic is, obviously, using the AU format, then it might be slightly more complicated (i.e. the bug in AU-specific, and it's IK Multimedia's fault).

Well, you've been using plugins or software instruments, to be exact, which have different formats: VST (or VST3) and AU. There's also AAX – plugin format specific to ProTools DAW.

SampleTank 4 is a plugin, which is installed in your system both as AU (Audio Unit) and VST to provide compatibility across various DAWs. Plugins are installed to this folder: [Your System HD] typically "Macintosh HD"/Library/Audio/Plugins. You may open it up and see a number of folders: Components, VST, VST3 and so on. Have a look in these folders, you may find a "SampleTank4.component" file in "Components" folder (that is SampleTank 4 AU) and also a "SampleTank4.vst" or .vst3 in a corresponding folder (that's a VST format of the same plugin).

Logic Pro is only compatible with Audio Units. So you might have tested a VST plugin with Reaper, and it works perfectly well. But the AU plugin in Logic might be faulty (well, we don't know that for sure still). That's what I was trying to tell above. Hope that clarifies it for you.

The SampleTank4.component and the SampleTank4.vst files are DIFFERENT files, though it's the same software instrument. So one of them might be corrupt, and another may work perfectly well.

When you say "I have always used SampleTank as vst" it doesn't mean that you used the VST format of the plugin in Logic, because Logic Pro is only compatible with Audio Unit plugin format. So it's the "SampleTankX.component" file that is loaded up when you choose the software instrument from the drop-down menu in Logic.

This is what I am trying to hammer down here: you might have tested a VST version of your plugin in Reaper and it worked well, but the AU version that you are trying to use in Logic malfunctions or isn't set up properly somehow.

See reply in the other thread.

Even with Logic?! Logic is only compatible with AU plugins, if I am not mistaken. VST is a different format.