OK, the secret about the misleading error message is disclosed. I threw in a trace command (in Mu_total()) and looked what values come in. root is really trying hard to find a solution, playing around x=9 (values are to interpreted as meters) eventually at the end goes havoc. Suddenly guess value number 6824 is 32.004 (out of the blue) and at the end guess no. 6848 is a complex number (32.004 + j*1.024...) and that is why the comparison is (correctly) throwing that error - we cannot compare a complex number to anoter for size, there is no linear order in the set of the complex numbers.
In case anybody is interested I attach the sheet (everything not necessary for the function to work was deleted) and the trace output. Of course I had to modify the function Mu_total a bit to make it work - replacing the last if by an otherwise statement.
