After completing a straightforward manual upgrade from v8.6 to v8.8 (nothing complicated), found that I can no longer save any changes to items in the Settings panel (Templates, Macros, Scripts, Styles, etc.) The "Save" button either endlessly spins or doesn't react at all. Observing the browser console, I keep getting these javascript errors every time a Setting item is either loaded or an attempt to save is made:

I've tried getting rid of the cache/TEMP folders in App_Data, reloading and rebuilding the memory/database caches, getting rid of all the cache in the browser, and stopping/starting the service on Azure, but nothing will get rid of this. Google search doesn't reveal anything useful to this particular situation.

Luckily I can still make and save changes in the Content/Media panels so that all works fine. But all the underlying items in Settings I can no longer edit. Need help to figure out how this happened. Any framework updates to this site are frozen in place until these errors are resolved.