Hi Mounira,
One of the possible reason behind this error coud be incorrect constraints. You can verify the inputs and constraints of your MPC problem as highly restrictive constraints can lead to a nearly singular Hessian matrix.You can also consider adding a small regularization term to the diagonal elements of the Hessian matrix. This process, known as regularization, can help improve the numerical stability of the problem.
Although changing the solver didn't work initially, but you can try using different QP solvers. Some solvers are better equipped to handle numerical issues than others. Exploring solvers with different algorithms might yield better results.
