An NADH:FMN oxidoreductase which supplies reduced FMN for the '4S' desulfurization pathway that removes covalently bound sulfur from dibenzothiophene (DBT) without breaking carbon-carbon bonds. Can also use FAD. Provides DszC and probably also DszA (DBT-monooxygenase and DBTO2-monooxygenase respectively) with reduced flavin (FMN and/or FAD). FMNH2 + NAD(+) = FMN + 2 H(+) + NADH FADH2 + NAD(+) = FAD + 2 H(+) + NADH Sulfur metabolism; dibenzothiophene degradation. Belongs to the non-flavoprotein flavin reductase family.