I remember there was a thread about this here but results were few.
Here is what I use in TelloFpv:
13: battery problem / incompatible
14: battery, but don't know exactly what it is
21: tello is tilted
24: IMU problem
29: Motor problem
203: Battery problem again
206: Tello not yet activated
I'm using the standard Tello app, version 184.108.40.206, firmware v01.04.91.01. This is just a guess, but it might have something to do with "activation." I know you have to activate the drone -- but only because I read about that on the internet -- yet the app has not asked me to "activate" my drone nor do I see any option to manually "activate" the drone.