Exactly the same here. Flew fine two days ago then upgraded firmware today. Every thing seem fine until drone restarted and missed out motor 2 on startup. It's also showing zeros in the diag page. Seems odd that 2 people have the same motor fault after firmware upgrade. 

I'm starting to think the motor problem isn't a firmware problem... but more of an update problem.. it started happening with 2.1.18 and then 2.1.20 and now this... i wonder if there's a fairly common fail in random update processes that cause this to happen... such a common occurrence over a spread of newer updates it's gadda be something in the update process itself... just thinking out loud

Could you make a checksum for the file?

I think the drone will perform a checksum prior to the actual updating.
I see that the drone runs 'checkplf' before updating.

@kornelius,

Could you check if there is any visible damage to the motor board (ESC) of motor 2, specifically the 3 MOSFETs?

Hmmm...

Try this:

Disconnect motors 1,3,4 from the center cross cables - to prevent any possible damage to the good motors

Connect motor 2 to either of the cables for motors 1,3,4 - rule out cable or motor position issue

Do a reset through the pinhole under the battery - I always do a pinhole reset prior to upgrading/downgrading firmware

Upgrade again to 2.2.3 using Freeflight 2.1.6 if possible, if not, just use the usual FTP method - hopefully this will reflash the motor board to 1.42

Check if motor 2 is initializing correctly - check if motor version is no longer 0.0

I don't guarantee it will work, but no harm in trying.

Good luck!

Kornelius do not strip your drone it isn't a hardware problem. I've rolled back to a previous firmware on my drone and motor 2 is functional again. Odd one this I must say . Parrot any idea what can stop one motor from upgrading itself but the other 3 to be ok ?

Now my drone is working with 1.41 motor firmware and 2.0.16 dron firmware
I swapped the No. 1 and 2 motors and waiting to charge the battery.
Then i'll try the upgrade again

Mine flew fine after the upgrade (and before for that matter even with the "bad" firmware) through about 4 batteries worth of time.  Then the right front motor (looking from the rear) went pfft.  Smoke came out and everything.  Now it won't initalize.  The other three try, but when the fourth won't, they all quit.  Still within the 60 days at Brookstone, so it's going back.

Parrot answered for my email (on support page, 02.aug):

"Hello ,

Could you please delete the application Ar Free Flight and download it again.
Deconnect and reconnect the WiFi from your mobile or tablette.
Put the battery full charge on The Drone and push the Reset button four seconde
Remove and replace the battery.
Reconnect the Wifi on the Drone and start the application Ar free Flight.
If you get the same problem please contact your dealer for swap your Parrot product.
Best regard ,"

The letter was received after that I sent the drone to dealer.

Sorry 

Update: Went to Brookstone last night - the store at which I purchased the drone...even the same salesman. although the receipt clearly states they want you to be happy, and will exchange or refund up to 30 days and exchange only up to 60 days, and I am still one week inside the 60 days, they WILL NOT exchange my drone with the defective motor.  Even though it clearly has no crash damage or user abuse. 
At the time of sale, they tried to get me to purchase the extended warrantee, and I asked if I had bought that, would it have made any difference.  They said no, they wouldn't honor that either.  Great Customer Service!  I will be contacting their headquarters this morning, and the CT Attorney General's Office as well.

If that PA applies to in-store purchases, I never was given a copy or signed one, and there is no such wording on the receipt.