Strange error on our webpage

Hi there

We have a strange problem on our webpage: https://patchwork.dk/ – the calendar function is not working on our frontpage, until we visit the page with the events: https://patchwork.dk/begivenheder-2/

We can change month, until we go to the page: https://patchwork.dk/begivenheder-2/ and then back to the frontpage – then we can change month.

It gives the error 503 or sometimes 404 in the error message.

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