and this happens on all of the machines for all users?
The extra code that you use like markercluster; do you use the API from google directly or do you use an offline copy hosted on your own server?
Yes, on all machines for all users. But for some users the problem started two days later. And sometimes it works but most of the time it doesn't. Till now we can't see any patterns.
For the add ons we use copies on our server.