So, I've finished to test all possibilities, and here are my conclusions :
- In locations.cfg, coordinates D (in degrees) must have no more than 6 decimal digits.
- In .mis file, coordinates M (in meters) mustn't have any decimal digits. Even if the converted number in degrees (D) has exactly 6 decimal digits.
As D should be equal to M/120000, the minimal move for a land unit is 3 meters (0.000025 or 25.10^-6 degrees), in other words M must be a multiple of 3.
But there is more !
When you convert the coordinate M (in meters) to D (in degrees), you must apply this formula :
for longitude : D = M/120000 - 6.10^-6
for latitude: D = M/120000 + 6.10^-6