I ran into following issue, soon after fresh install on Ubuntu 18.04 LTS Cloud server with V11 stable branch of Frappe/ERPNext.
Installation completed without any issues, but during setup wizard itself, while uploading logo errors were visible inside browser console.
WebSocket connection to 'wss://crm.site.ext/socket.io/?EIO=3&transport=websocket&sid=lLRcThF1iZ4deOzLAAAB' failed: Error in connection establishment: net::ERR_NAME_NOT_RESOLVED
I had SSL installed and ERPNext site was created with same name as domain itself. (crm.site.ext)
Update #1: This turned out to be OS Configuration issue, not directly related to Frappe/ERPNext.
Update #2: Unfortunately problem returned after sometime.