Unknown error for new visitors

On this website, I have access to all agenda. When I open a private window, I’ve got an “unknown error” instead. A 60mn cache is activated. There are 7 agenda showing.
If I unselect all agenda, save and put them back, it all goes back to normal use for a few days.
It makes no sense.

The error on the console is : “TypeError: right-hand side of ‘in’ should be an object, got number”
From this part of your code :

xhr.onload = function (eLoad) {
try {
var response = JSON.parse(this.response);
if (“error” in response) {
throw response;

It makes no sense to me. Specially the part when it works and doesn’t work at the same time depending on if you saw the site before it broke or not.

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