Levenberg-Marquardt, error

Hello all, I'm currently trying to implement a nonlinear curve-fit routine in LabView (Nonlinear Curve Fit.vi, Levenberg-Marquardt). I'm trying to find a best fit for a given exponential decay, and the fitting model is passed to the Nonlinear Curve Fit VI through a VI. However, I don't want to implement the model as simple function, but instead I want to describe the model with a Runge-Kutta ODE solver. I do that, so I can later on implement more complex mechanism and models that do not have an analytical solution. When i try to run the optimization the Levenberg-Marquardt algorithms runs a few times, and then returns "Error -20041 Analysis: The system of equations cannot be solved because the input matrix is singular." This error seems to occur quite often, but the other topics here couldn't solve my problem unfortunately. Any suggestions on how to solve the problem are highly appreciated