Cannot Publish Events

I just relaunched a site with a new theme and other improvements. Now, I cannot publish events. I can publish pages and posts, just not events.

I get this error:

Your event details are incorrect and cannot be published, please correct these errors first:
Something went wrong saving your event to the index table. Please inform a site administrator about this.

I’ve worked through the suggestions I found in these support topics, but it’s still not working (trying a default theme, deactivating all plugins, etc.):

https://wordpress.org/support/topic/cant-change-event-status-from-draft-to-published/
https://wordpress.org/support/topic/error-when-saving-new-events/

I have the old site it is based on still running and publishing works.

We can work around it by going to the All Events Page, check the ones we want and do a group edit to publish them. But that is not ideal.

Any ideas?

The page I need help with: [log in to see the link]