CGS solver fail to solve the system => try with Gauss solver (longer but exact)

Hi, SPIS community!
I’m running a simulation, and for the first 2-3 seconds, everything seems fine as I get “Conjugate Gradient(generic, for matrix inversion) reached convergence at iteration number” messages, and I see the simulation running without problems. However, after a while, the console gives the following error message “CGS solver fail to solve the system => try with Gauss solver (longer but exact)”.
Where could the problem be? I’ve run other simulations, but I’ve never seen this message before.

Thanks to anyone who can give me any advice.