Theoretical uncertainties in exclusive electroproduction of S-wave heavy quarkonia Jan Cepila, Jan Nemchik, Michal Krelina and Roman Pasechnik Eur. Phys. J. C, 79 6 (2019) 495 Published online: 12 June 2019 DOI: 10.1140/epjc/s10052-019-7016-9