If indeed you can plug that same XLR cable into any other XLR audio on the board and it works fine I’m going with a broken XLR as well. Try plugging in to the mix 2 jack and with the speaker humming try moving the connector around some and see that make the hum
stop.
It could be a broken connection where the XLR jack is soldered to the circuit board and moving it some may cause the hum to stop….at least while you hold the connector!
My guess is pin 2 or 3 on the mix 2 XLR jack are open either from a broken jack or in the actual circuit itself.