It doesn’t seem to be a standard connection error, you get some spinning gems and some server can’t be reached message for that. It showed up roughly a year ago, I made a bug report about it back then.
It feels like I get the “Unknown Error” more often (or possibly even only) when connected to an active global chat, meaning 001. Wild guess, the client is trying to fetch some chat information after battles on top of reporting the battle results, then fails due to having been disconnected from the chat channel. Since the game related communication went through it doesn’t have a proper way to react.