Hey all,
Some time late july 2020, I turned my Mavic Mini on only to be greeted with the same error message as you all.
I spent the past months looking all over forums for answers (including this one) and decided I would proceed to change the ESC chip.
I did so yet the error code remained.
The ESC chip was therefore not the problem.
In a desperate last ditch attempt to diagnose the issue before scrapping my Mavic ;( or paying a hefty price to have it repaired without warranty, I completely tore it down yet again and inspected every nook and cranny.
I discovered that one of the coils in the top left motor had its copper wire ever so slightly proturding.
I fiddled around with it using tweezers and managed to set it back into the place I believed it should be (in order for the coil to function correctly and have electric current flow through it to generate the magnetic field needed to turn the propellors).
Somehow this worked, the Mavic Mini finally turned on without errors.
I am therefore adding this reply to make you all aware that something as little as a protruding copper wire in one of the motor coils may be the reason for this Error 30210 code showing up.
TLDR;
My advice : Before buying a replacement ESC microchip, inspect every coil of every motor, making sure they look as they should (tidy with no protruding wires), and if you do find protruding wires, attempt to reinsert them into the coil with tweezers. If that doesn't work then go for an ESC chip replacement.