Also, if I quit MC24, I cannot relaunch it without logging out or restarting. MC23 has no issues.
--> resolved - per the forum re: MC24 crashes, I enabled MC24 in System Preferences/Privacy/Accessibility. MC24 can be relaunched; MC23 did not need this.
MC24 doesn't need that setting. At least not for this issue, I think.
What was more likely was that MC24 was running an Auto Import in the background, or building Thumbnails, so it didn't completely shut down, but was running those processes in the background. I don't have a Mac anymore and don't remember how to do it, but if you looked in the Mac equivalent of Windows Task Manager you would probably have found a MC24 task running in there.
As you didn't have that setting for Media Network set in MC24, it sounds like the upgrade didn't find your MC23 Library, and you didn't find a Library Backup from MC23 and Restore it with Settings selected in MC24. As you have only just upgraded, that is what you should do now, to get all your settings, views, etc. from MC23 in MC24. Just backup MC24 manually first in case something goes wrong, then find the most recent MC23 backup and Restore it.