 In the 1998–99 season, he was an important part of the Varteks team that reached the quarter-finals of the now-defunct UEFA Cup Winners' Cup, appearing in all of the six matches played by the team in the competition that season, as well as scoring three goals in the two matches against SC Heerenveen in the second round of the competition.After grabbing a 5–4 aggregate win against the Dutch side in the extra time of the second leg at home in Varaždin, Varteks faced RCD Mallorca in the quarter-finals, losing the return leg 3–1 after a goalless draw at home in the first leg.In the summer of 1999, Mumlek left Varteks for Dinamo Zagreb (then known as Croatia Zagreb) and went on to make four appearances for the club in the UEFA Champions League group stage, appearing in both home and away fixtures against Manchester United and Olympique de Marseille before Dinamo were knocked out of the competition after finishing 4th out of 4 teams in their group, one point behind the 3rd-place Sturm Graz.