It's pretty hard to tell ☒ . During the discovery of creating this episode, I got that kind of message a few times, but nothing in the error pointed me in the right direction to resolve it. I searched the Apple developer forums a few times and found several hints to resolve the issue and one of them finally worked. However, the kind of error message I got was with the handshake and it prevented authenticating the user. I haven't seen this particular case where you are authenticated, but still get that flash message.
☒ You shouldn't have to use Bootstrap with FullCalendar. This issue that you're experiencing is likely due to the changes in Stimulus. They changed the path to "@hotwired/stimulus" instead of "stimulus" when importing the library.
☒ I'd say for a Rails 5.2 application, this is still probably along the path that you would have to go. I think that it was Rails 6 that added environment based encrypted credentials.
If you're using webpacker currently, then I would still go that route. If you're just starting out a new application, I would probably go the esbuild route from jsbundling-rails. This is going to be one of the more supported options in the long term.
