Please help diagnose custom post type permalink problem

I’m appealing for help diagnosing a problem with a plugin that uses custom post types. This is the worst type of problem to solve – an intermittent one. The plugin is working fine for me and for a couple of other people who have tried it. However, I’ve had two people report that they get 404 errors when they try click on the permalinks associated with the posts.

This is an event management plugin. At one point, I thought the ‘event’ slug I was using for my custom post type might be clashing with categories or pages in the permalink hierarchy tagged ‘event.’ However, I’ve changed the slug from ‘event’ to ‘rsvpmaker’ and that doesn’t seem to have solved the problem.

The developer I’ve been corresponding with says it works on one of the WordPress sites she is managing but not the other. I haven’t been able to figure out what the difference might be. I had her do a data dump of the contents of the $wp_rewrite object and confirmed that the array of rewrite directives is being generated properly on the problem system.

Any ideas on how to track this down?

The plugin is posted here:
https://www.ads-software.com/extend/plugins/rsvpmaker/

The register_post_type code is here:
https://wordpress.pastebin.com/ywJafrx3

I include add_action('admin_init', 'flush_rewrite_rules'); so the problem is not that the rewrite rules need to be regenerated.

Any suggestions about how to figure this out would be appreciated.