Kontakt 5 is not supported in Catalina.

https://support.native-instruments.com/hc/en-us/community/posts/360008490998-Will-KONTAKT-5-be-compatible-with-MacOS-Catalina-in-the-future-en-us-

I have no problem with Kontakt 6.

Hi Mike - thanks for weighing in.

Not Kontakt 5 - my mistake. It's Kontakt 6.

Funny thing, it works in a new user account - just not in my original home account.
Worked before the upgrade, though - now I have to go through some troubleshooting, but not sure where to start.
I suspect a clean install of Finale might help - I wish there was some way of telling what was causing the conflict.
I bet there is, but I haven't a clue where to start.

Console.app is your friend. It timestamps everything your Mac does. You are looking for something trying to load and failing or something that shouldn't be loading but is. 

You want to isolate the few seconds beginning when you hit Play. Drill down on each event. Hopefully, you'll hit on something that you recognize. I've had Finale 2009 Help files and HP print drivers gum up the works—removing ancient code fixed me up. Other times it's old freeware that hasn't worked since OS 10.6 — you never know. I once solved a whole mess o' problems by removing an old Soundflower .kxt.

Purging older versions of Finale and Kontakt is a very good idea — they don't work anyway. Sometimes, you'll clean out enough and things start working again. In that case, declare victory and move on.

Mike, do kexts reside anywhere besides HD>Library>Extensions? Apple had suggested them as a possible culprit, but looking at the set from the working account and the failing account - they're identical. They both have the same creation date for the Soundflower kext - 2014.

I'm gonna try deleting the .fprf and .plist files next... see if that does it... they're user-level....