Details here: https://github.com/spesmilo/electrum/issues/4968
copy paste from SomberNight in the github issue:
TL;DR: There is an ongoing attack against users where servers raise exceptions when a client broadcasts a transaction; in this case the error text is displayed as is in the client GUI. The attacker has spawned lots of servers on different /16 IPv4s to increase his chances of being connected to. The error messages are trying to get the user to download and install malware (disguised as updated versions of electrum).`
apparently previous Electrum versions allow richtext in error messages sent by the server, so the upgrade warning might look legit to people…
Always remember to verify pgp / gpg signatures of bitcoin software you install.