The drag force of a translating sphere half immersed on a flat liquid-gas interface in the presence of insoluble surfactants is analyzed. Special attention is paid to some singularities of the perturbed adsorption GAMMA and of the drag force. It is shown that in the frame of Stokes hydrodynamics the steady-state perturbation GAMMA does not vanish at infinity but tends to constant and the drag force due to the Marangoni effect is infinite. These discrepancies are explained considering the incorrectness of the Stokes velocity field far from the sphere. The Oseen approximation is proposed as a better hydrodynamic model and, as a result, a finite force proportional to the logarithm of the Reynolds number is obtained.