Unable to import .ics files into Calendar on macOS 15.4.1

I regularly download .ics files from meeting providers: healthcare, business-related, webcasters.

For a while, I've been getting an error notification in Calendar, but the app retained the event. As of today, those events just vanish a moment after I import them.

I download the .ics file to my Downloads folder.

Then I try opening the .ics files: the OS chooses Calendar as the app to use. I close the details window for the event and all is well for about five seconds, then the event evaporates.

I tried the Import function with the same results.

Anyone else having this problem?

[Re-Titled by Moderator]