The tapping procedure didn't erroneously break into a closeby internal oil port or pathway, did it? In other words...a tap or drill longer than needed for the drain plug being inserted into and through the targeted plug threads ...invading and penetrating further inside, breaking or penetrating into the wall of an oil port of some sort? Was any drilling done through the drain plug opening during thread repair?
I'd think you have some disassembly and very careful, fine inspection, inboard through that plug opening. A micro camera type device may even save some tear down.
'95 Ducati 900SS/SP
'07 Ducati SS800
'98 Honda VFR800 FI