Translate RSVP NOW!

Hi there,
I’ve translated the plugin Event Tickets (a plugin by EventBrite). But as you can see on the frontend it says ‘x Spots left’ and RSVP NOW!

I believe that after a WP update my Dutch translations ‘Beschikbaar’ and ‘Reserveer’ are gone, despite i can find them in the Loco translation.
Somehow it doesn’t take effect on the front end.

I’ve also tried with uploading the mo and po files to the wp-content/languages/plugins and /wp-content/plugins/event-tickets folder, to no effect.

What to do??? How do you handle translations if you don’t use Loco Translate?
Rob

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