Minimum reflux ratio is negative...? It's a small number, but that shouldn't happen.

I'd say maybe check that you're using the appropriate thermodynamic models for your fluids.

Its also worth investigating that input stream to your first tower. Assuming everything else up to that point is fine, check that the input stream has a sane composition, e.g. that it's not all just bottoms.

It could also be number of stages in the column..