Most likely the intention of the check was to ensure that all residue interactions are covered, which makes only somehow sense for complete protein self-interactions. For interactions between two proteins the check will fail.
The line will be removed in the analysis modules fda_graph, fda_shortest_path, and fda_show_stress.
https://github.com/HITS-MBM/gromacs-fda/blob/99369234ef2ca56f803a0628cad98011aff0726a/src/gromacs/gmxana/gmx_fda_graph.cpp#L119
Most likely the intention of the check was to ensure that all residue interactions are covered, which makes only somehow sense for complete protein self-interactions. For interactions between two proteins the check will fail.
The line will be removed in the analysis modules fda_graph, fda_shortest_path, and fda_show_stress.