If you are using a middle-tier try to connect to the server directly as recommended here: https://world.episerver.com/documentation/developer-guides/CMS/user-interface/websocket-support/ - there's a chance that the issue is caused a VPN or some middle-tier.
I am working on an episerver dxc site and we are getting 416 erros in the console when logged into the cms. we also have the issue with web sockets but when i disable web sockets i am still seeing the 416 errors so i don't think they are related.
the result of this error is that when we add media or blocks to the site they can be saved and published but we can not see them in the right menu under the blocks or media tabs. i can add folders but when i add a block to a folder the folder remains empty.
this is issue is not seen when running the site locally. the issue comes up on both the integration and pre-production environments.
the error message is:
"Unable to load /EPiServer/cms/Stores/contentstructure/ status: 416"
statusText: "Requested range not satisfiable"