Third club spoiled visiting support

Next week the first half of last qualifying phase will be played, before entering group group stages. Along with the Europa, Champions league will start first and one of the pairs are real champions in its own countries.
First duel of Olympiacos and Rijeka will be played on the second day. But, this Thursday both clubs (surprisingly) has decided that no visiting fans will travel either to Croatia or Greece: nor Croatian supporters should travel to Piraeus nor Greek fans will go to Rijeka.
Motive of this (strange) decision is due to security concerns and fear of clashes before, during and after the game at their stadiums. Also, selling tickets to the guests will be strictly forbidden and rigorously controlled. However, the main reason who is damaging this fun is a Serbian club. Supporters from Crvena Zvezda, who are in excellent relations with Olympiacos fans, might come and cause a ruckus in both matches.