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Have you any idea on what is going on?

Apart from instances in the fairychess include file, I see one instance of onboard in your code. It appears in your stalemated subroutine, and the variable passed to it has previously been calculated from the -Range function of a particular piece that has been found on the board. So, the problem may be due to the output of a particular range function. One -Range function that looks suspicious is your None-Range function, which returns an array of two empty arrays. It might work if you change this to just return an empty array, as a -Range function is supposed to return an array of coordinates, not an array of arrays. If that doesn't fix the problem, you can put in some debugging code to find out which -Range function is giving bad output when the code stops working.