This indicates that your webserver does either not support websockets or fails to answer in time for whatever reason. If you have a proxy server in front of your docker-compose stack, ensure that it also supports websockets and routes it cleanly to the docker-compose.
Zammad falls back to Ajax-Longpolling to still enable your users to get close to realtime updates.
It’s not Zammad related, check your webserver logs which may help with finding the issue.
This can be a normal error.
This usually happens if you run Zammad for a very long time without reload and collected that much data that it’s getting too much for the Browser. (Browsers limit the local storage for various reasons).
Reloading should help to make this message disappear.
If not, you possibly have a fairly big ticket with big images and stuff and you’re loading which could lead to this issue.