etherpad-lite: iframe error

I noticed an iframe error with different pads from different server in different browsers (Firefox, Chromium). Cache is empty.

https://pad.colibris-outilslibres.org/

https://pad.kits.blog

The browser console shows me following

TypeError: Cannot read property 'setStateIdle' of null

For pad.kits.blog nginx is properly configured:

proxy_pass http://localhost:9001;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "Upgrade";
proxy_set_header Host $host;
proxy_set_header X-Real-IP         $remote_addr;
proxy_set_header X-Forwarded-For   $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;

And I haven’t found any websocket error.

https://video.etherpad.com works 😉