On a 2020 Nissan Leaf S PLUS, which is leased in December 2020, I had the same message "Service EV System - Unable to restart after power off" back on November 22 at just over 33,000 miles.
To everyone who might Google this in the future, I would like to point that there appear to be 2 different error messages. There also appears to be one which is "Service EV System - No Power" and that appears to be remedied relatively quickly and easily by getting a new 12V battery. But "Service EV System - Unable to restart after power off" is the message which appears to be caused by a bad battery cell in the main traction battery.
Fortunately when my car did not want to start again on November 22 I was able to use Nissan's Roadside Assistance for free, to be towed to a dealership, since I was still within the 3 year 36,000 mile warranty.
Getting a loaner vehicle required a lot of calls to the dealership, as it was just before Thanksgiving and all their loaners were with customers and Hertz and Enterprise were not having any Nissans available either which they could give to me. Through my persistence I ended up getting a loaner, funnily a Murano so now I spend $50/week for gas to get to work while with the Leaf it was much less or free.
Yesterday as the vehicle was in service for 2 weeks and 2 days already, they told me how the Leaf tech is still in the process of checking all the individual battery modules. They said that the Leaf tech also has to work on other urgent repair jobs inbetween, which is why it takes longer. I was informed in case it might be just a module or two which needs to be replaced, then it would be a quicker process to finish the repair, but in case a whole replacement battery would have to be ordered from Nissan, that would take months. They brought me into the back of the service area, and showed me how they had the Leaf battery lifted up there, coned off, so that the Leaf tech could work on checking the modules.
I am grateful that I received a loaner for free, so I can still get to work, at the same time I am also frustrated by the slow pace of the diagnostics process.
This is my 4th Nissan Leaf (2012, 2016, 2018 and now a 2020), and I never had any major problem. Recently even my brother got a Leaf, and I wasn't able to tell him about any problem I had with any Leaf. Now I am not sure to what extent I could recommend the Leaf to anyone; I understand how others can recommend the vehicle when they had no problem, but in my shoes now it feels different.
While this thread is about 2018-2019 in the title, I just wanted to add to it that obviously this issue can affect 2020 Leafs as well.
Finally my question: for anyone who had this issue before that a cell needed to be replaced, did the bad battery cell issue ever pop back up again later?
The reason I ask is that I actually bought out the lease on the vehicle before the problem happened, and now I am seriously thinking about to what extent I should buy the Nissan original Nissan Security+PLUS extended warranty with 8 years 120,000 miles. As far as I remember, there is a dealership from California which sells those for a lower price over the internet.
It appears that without the extended warranty, one might not get a loaner in cases like this. And in case the inverter stops working or the high voltage system and it is out of warranty coverage, I don't want to see how much it would cost to have the shop work on it for days or weeks on end when one might have to pay out of pocket for it.