Hi! I was flying yesterday and suddenly one motor stopped and drone fell on the ground downwards. Like height of 50cm. Then drone didn't work and controller says ESC ERROR 30085. Now when I start up drone, one motor does not make that starting dance at all and the motor feels stiffer than others, when spinning it with hands. When I looked closly to the motor, there is like something black, maybe burnt?

Any advice? Is ot ESC or is it motor? Should I buy a new motor with arm?

Hi, there. We are sorry for any inconvenience caused.
For your problem, it is recommended to send us your drone, you can click this link to start a case: https://repair.dji.com/repair/index

Time to replace that motor.  It is not really hard to do.  Check on YT for some videos on it.  

DowntownRDB Posted at 1-22 05:37
Time to replace that motor.  It is not really hard to do.  Check on YT for some videos on it.

Do I have to replace whole arm, or can I replace only motor?

Veiva Posted at 1-22 05:50
Do I have to replace whole arm, or can I replace only motor?

I've done it both ways and it is much easier to just replace arm and motor.  They are fairly cheap on Aliexpress.