Some believe this is directly to a Steam client update which broke EAC and was reverted.
My own experience is that the error relates to the “Official Realm”. I am able to get on the “Modded Realm” with no issues BUT on the “Official Realm” the error occurs.
Note: Have validated files (no errors), repaired EAC, uninstalled, re- installed PC, Rebooted, Steam, PC and router. Steam was tested on the client update (pre-revert) and after reverting.