I just accidentally briefly plugged the usb cable into the MIDI port

Mark II units have a PTC fuse on the CAT-5 port. Although this doesn't guarantee that you won't damage anything by plugging a USB plug into the CAT-5 port, it does prevent things going up in smoke and causing lots of damage.
 
My Axe FX II, Mark I has a piece of black duct tape on it over the MFC jack - a prophilactic, if you will, for my protection against accidental penetration.
 
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