Mosquito working, but MQTT explorer can't connect

I’m running Home Assistant OS 13.2 with Mosquito on a Pi 4, which I use to handle some Tasmota devices via MQTT. They work fine, but I recently tried connecting to my HA box using MQTT explorer on my Mac. I got a “Connection Refused: Not Authorized” message.

I’m using the exact same credentials used by my Tasmota devices, and they work fine. So what am I doing wrong?