I'm getting the exact same problem on my 2011 Leaf. In my case, I didn't drive it for a couple of weeks and during that time the 12v battery went flat. After charging it up all was fine except I noticed the yellow warning light on the dash, and LeafSpy is now reporting the "C1A66 0109 BRAKE Master Pressure Sensor BR-89 code". I can't clear the code either, so I'm presuming that I won't be able to do this until the problem is resolved. The car drives fine (in brake fail-safe mode I suppose) but you can definitely notice that brake regen is gone - engine regen is unaffected.
I haven't taken the car to my dealer yet, but my working assumption is that the brake pressure sensor needs replaced. I live in the UK and unfortunately this part is over £500 including tax...that's not for the whole master cylinder, just the sensor! There don't seem to be any aftermarket alternatives, hence the high prices I suppose. Shame on Nissan for price gouging though, when the same part is available in the US for $300 or so, which is well under half the UK price. I'm considering taking a risk and importing the part from eBay and trying to fit myself, but I'm not much of a mechanic and the brake cylinder doesn't look that easy to get at. I might find an independent garage to do it for me. This whole episode is a bit annoying seeing as my car is not yet 4yrs old with only 23k miles, and actually drives fine without brake regen...and if I pay dealer prices to repair this fault, the recovered electricity during the remaining life of the car will come nowhere near to approaching the repair cost! I wouldn't bother except I won't be able to sell the car unless I fix it!