Thanks for the reply, and your questions.
What I meant by "not recognizing its own plugins": First, opening Logic is problematic in that the process stalls (not a crash or a freeze) during this part -- Scanning Audio Units (finalized 0 or 1). It stays there for several minutes. No progress shows in the progress bar. I've let it be in that (what I perceive as stalled) activity until I abort in order to open the program.
When I then open an old (and we're talking old as in things I worked on as recently as earlier this week from a previous version of Logic (10.4.2) and a different computer) and finally get a project open, the Logic plugins -- Amp Designer, for example -- will not load into the stem that it is used on. In the plug-in manager, the Logic plugins have not been "successfully verified". I have rescanned, restarted numerous times. Same result.
So, sounds like your way of explaining the issue is correct.
However, I'd be super shocked if it was system overload issue. Running High Sierra on mid-2011 iMac, 3.4 GHz i7, 16gb RAM, 1TB ssd. There is 870gb empty space. I just got the clean used iMac.
I am suspecting it has something to do with the migration from my MacBook? I migrated everything from it prior to downloading Logic, earlier this week. And on the MacBook, I kept Logic audio files (and most of my Logic projects) on an external storage drive. This morning I removed the external storage drive and Logic asked if I wanted to download the Logic audio files onto the computer rather than search for external, etc. I clicked yes, so it installed.
Still same issue, though. I can't open the old project files stored on my external drive. Logic just stalls searching for audio units (its own).
IMPORTANT DISCOVERY: I have a handful of old projects that were stored on the previous computer that migrated to MUSIC folder in the current iMac. Those open and play fine. Oddly, the plugins still read as (seemingly?) not validated.
So it's feeling like my iMac is not liking the external drive?