Richard,
thanks a lot. I rewrote Minerr with constraints as a function. That worked. Your comment about how the constraints are handled are also very helpful.
However, I had to use the residual sum of squares as objective function that was minimized. The vector of residuals that you proposed as superior method did not give give satisfactory results with Minerr. Also, your short-hand version for generating the vector of residuals did not work. I had to program a subroutine with a for-loop to generate the vector.
Again thanks so much for your help. I got a functioning program.
Ulrich