Failure to extrude in one area of build plate only

Hi all.
A mysterious problem just started with my Up Box+ s/n 531237, and I need advice. Suddenly, the extruder will not print on the front left corner portion (approx 1/9th) of the build plate! I can hear the extruder gear sounding like the filament is jammed, but the clearance was just set correctly, and extruding begins fine once the head leaves that area. I took a temperature measurement and that area is the same as everywhere else. The manual bed-leveling routine works perfectly, including a stop for measurement in that same, left front-most, 1/9th section…! I’m using UpStudio 2.6.49.627. I had just cleaned the extruder gears, replaced a build plate, and leveled the bed, after which, this happened. Can you help diagnose the problem?? It seems one other person has experienced the same failure, and I’m pursuing the solution with him, but I wanted to see if anyone else can offer guidance to fixing the problem? TIA
Steve