I assume you know but in case others don't, is same thing as going to network setting in bitcoin-qt and then checking "deny incoming connections". This setting is about if you want your node to be public for everyone to use to use or if you want it to be private for yourself only.

If the solution was making bitcoin core a public server then that would be a bit contradictory to romanz/electrs whos selling point is it's made to be a private personal electrum server even though it can also handle being public if you configure it that way.

Fortunately/unfortunately changing listen to 1 did not solve the error.