Stuck on month

I’m running WP 5.4.1 and The Events Calendar 5.1.1.
The theme is called “Ed School”

My problem: I have a page with my events calendar –

https://www.volksschule-herbststrasse.at/events/

– with default view set to “month.” When you click on the left/right arrows, the page reloads but you stay on the same month. But the URL changes to

https://www.volksschule-herbststrasse.at/veranstaltungen/monat/

My site is set to German, and “veranstaltungen” is “events” in German. Maybe this is part of the problem. I tried setting the site language to English and got the same problem, but when I click on the month navigation arrow and the page reloads, still showing the same month, the URL now looks like this:

https://www.volksschule-herbststrasse.at/events/(/?:month|monat)/

The same thing happens (view stays on same month but URL chagnes to the one above) when I try to change the calendar view from month to day or list.

Strangely, clicking on the name of the month and navigating to another month from the drop-down menu works, but only when I am logged onto wordpress.

What I have tried:
-deactivating all plug-ins except The Events Calendar
-switching to 2016 and 2020 themes (with plug-ins deactivated)
-changing most of the settings on The Events Calendar

none of this stuff worked. Any help would be appreciated!

Best, David

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