Same issue here; windows defender flags the latest update as a virus... First time this has ever happened to me, though.
Edit: disabling windows defender and trying to install the update doesn't seem to work either, since the installer throws constant file writing errors...
Edit 2: managed to install the update. Seems that MB did not close fully when i clicked the player's close button, and was still kept running in the background. Had to fully close it using Windows task manager before the installer worked...