This is a new one for me...

I haven't flown my M600/RTK for a while now, mainly due to winter weather, so I connected the M600 to DJI Assistant 2 (V1.1.7) and upgrade to the latest firmware:
- Lightbridge 2: V1.1.60 (2017.04.20)
- M600/Pro: V1.00.01.65 (2017.10.24)
- RTK (air side): V1.0.0.21 (A3/M600/M600PRO) (2017.2.17)
- RTK (ground side): is the same as the RTK airside.

All Assistant 2 RTK Antenna position information is correct.

Everything seem to power up fine on my RTK ground unit and slow flashing green LED on the M600 (after recording home location).
DJI GO App seemed happy with a "Ready for Takeoff" green bar at the top.

After about 15 seconds in the air I get a solid red LED and the message "MC : [RTK] mounting exception" (see attachment).

I have successfully flown many-many times with the configuration and have not modified anything at all.

Ideas?

We have two possibility to mounting, forward-backward and side-by-side, maybe is the problem or X,Y and Z is being measure wrong.Third possibility is ANT1 and ANT2 is reversed

Problem solved!
This fixed was very technical and actually has a name, due to it's frequency, it's called...Finger Trouble!

During one of my last firmware upgrades all the A3 settings were restored to factory defaults.
I had all the screen shots of each M600 and LB2 page but when I re-entered everything, primarily the RTK numbers, I accidently reversed the two ANT offsets making ANT 1 in the back and ANT 2 in the front. Once I switch the two X axis value test flight was rock solid again.

Happy.

Nice try, glad to know you figured out the problem, hope it is useful for other users who may have the same problem, thanks for sharing.