I don't have any grand advice. Likely there is something odd about the ground or neutral of the troublesome outlet. Could the fridge frame be intermittently touching another ground? Measure the resistance from each pin in the power plug to the frame. Do you know if the trip occurs at startup?

The long extension cord will lower the current draw somewhat.

It might be interesting to measure voltage between pins on the problem outlet and the same pin on another outlet while the fridge is starting. (be safe while doing this)