Hm. There is at least one library file InDesign CC 2019 should be able to open.
It comes with every InDesign installation and did not change for years:
/Presets/button library/ButtonLibrary.indl
Direct access to that library is through the Buttons and Formfields panel.
From my German InDesign CC 2019:

One could try to move out ButtonLibrary.indl from its folder and replace it with the version that comes with InDesign CC 2018.1.
If you are able to open the version from CC 2018.1 you have no technical problem with indl files per se.
Regards,
Uwe
The system does not allow me to remove that file since I am not an administrator. Anyway, the buttons panel is darkened and does not offer access to the library. Seems like a dead-end, at least for me.
But its all moot now. I just clicked on the library now and it magically opened! Perhaps shutting down on Friday and starting this morning helped it take effect, as opposed to normal restarts I undertook last week.