hi @filz51, turns out I can’t reproduce this :/
I was looking at April 2016 in German in my comment above, turns out April is the same in German! Now I’m checking May and it correctly loads as ‘Mai’ using previous setup I had to ‘reproduce’ the bug.
I’m looking closer at your site now and I see that there is a JS error, and it’s related to the calendar plugin:
“Uncaught ReferenceError: moment is not defined”.
Also, I get the issue in all browsers, so my guess is it’s a specific setup you have in Autoptimize and the reason you see it in some browsers is because you’re not logged in as an admin on those browsers.
Therefore, we’re back to you testing your autoptimize settings.
@beheard – We include one JS file as well as depending on WP’s jQuery:
/wp-content/plugins/wp-fullcalendar/includes/js/main.js
If you can maybe provide a screenshot of your settings, maybe we can reproduce it, otherwise try adding main.js as an excluded autoptimize file.