baldguy on "[Plugin: All-in-One Event Calendar] Can't change calendar theme"

AI1EC version 2.4.1

No matter which theme I try to activate (my custom one, or the included ones), the site will not activate anything other than Vortex. I click "activate" and I am redirected to the "All Events" page. When I go back to "Calendar Themes", Vortex is still selected.

Perhaps related, I also experience this redirection when I save "Settings" - I am redirected back to the "All Events" screen, but the settings have been saved.

I've installed this on two different sites, and the behavior is the same. Fresh installations of 2.4.1 on new WP 4.5.2 site. Changing WP theme has no effect. Disabling all plugins has no effect.

https://wordpress.org/plugins/all-in-one-event-calendar/