I have found several incarnations of this issue across the forums but not quite the same as the issue I'm experiencing.
I installed an update approximately two weeks ago which took nearly a whole day to download which was very unusual.
A few days after, I switched my phone on to be presented with the "Moisture has been detected in the USB/Charging/port" notification.
My phone has not been anywhere near any liquids or moist/humid conditions.
I had to reboot my phone multiple times for it to go away and allow cable charging.
Strangely, it has only happened since when trying to plug the phone in to the Gear VR headset.
Does anyone have any advice other that drying the port or using the safe mode trick?
As this seems to have happened after the unusually long update and only when connected to Gear VR, I'm convinced it's a genuine error with the hardware.