Just in passing what version of mqtt are you running? There's been a little of discussion about mqtt explorer not being compatible with the latest mqtt 5 ( I think) protocol
Which version of MQTT Explorer are you on? You might want to note that someone has picked up development and there is a "beta" version that has recently been updated, that version had been knocking around for a long time until someone started work on it again. I'm on the latest version, it seems to work fine.