HERE is the answer as posted with the question as to why Plex quit working (as outlined here) after the last update. (see link posted above)
As with most things, when the resolve is found, the issue becomes clear...
The dysfunction with Plex did not clear-up after yet ANOTHER update (3.37.2 to 3.39.5 from the LimeTech repository). It got resolved when unRAID was subsequently updated from 6.4.1 to 6.5.0 and THEN Plex resumed normal functioning...
So, what does this tell us?
1) As I contended, it was a 'coding' issue - a Plex update was applied and functionality went "belly-up" as outlined (extensively) in these discussions; it should also be noted that this is not the first time a dysfunction has occurred upon applying an update; previously, Plex could not find the "server" (itself) after an update was applied. This is significant as you will see in the continuing discourse - it is indicative of improper testing before release, in particular, on the given platform in question, unRAID.
2) As I also noted, NO CHANGES to Plex settings had been made prior to the onset of dysfuntion; it was entirely elicited through the application of the update. By the same token, NO CHANGES that could be applied by the user were EVER going to rectify the dysfunction. Until the disconnect between Plex and unRAID was resolved through proper handling within the code, the issue was to remain unresolved.
In this case, the proof certainly is "in the pudding" as far as what it took to become functional once again.
3) The disconnect in this case is further emphasized by another fact - the dysfunction with Plex had gone on for so long that I started [was forced into] using a functional program called "Emby". It, in fact, was completely operational in the collection and storage of data from the VERY SAME movie data base source as specified in Plex and was fully functional DURING THE ENTIRE TIME of the problems with Plex.
So, what is tells us is that changes made with Plex (update) threw the works out of kilter in relation to the platform it was operating under (which may not have been an issue on other platforms) AND until the coding was properly realigned (with the unRAID update), it remained dysfunctional, even through a number of its (Plex's) own updates.
By implication, it also tells us the updates are not being completely or properly tested on the platforms to which they apply, in this case unRAID... not exactly an 'unknown' quantity.
And, at this point (for over a week now), it is working better than "ever"; not only in the restoration of functionality that went "out the window", but I am actually getting metadata on the cast that has been missing for so... long that I can't remember.
Enough said. We're "back in the saddle again". I suggest the Plex pay heed to what has been presented in this issue in order to avoid future problems, the potential of alienating the user base and the needless aggravation it presents to the Plex Community.