Scheduling conflicts are alluded to as a possible reason.
The article was published on STORY's website on 15 September 2017
This translation is not the best (Google does have issues with Croatian, but you can get the gist):
As Story says, singer Jelena Rozga and cellist Stjepan Hauser are no longer together. After two and a half years of connection, 10 days ago, they made the decision to each go their own way. The pair has decided to keep the reasons and the details of the break to themselves, and despite the fact that their love came to an end, there were numerous good times together, and will remain amicable.
Jelena and Stjepan were seen in public for the first time at the 2Cellos concert in Ljubljana in July 2015 when they were photographed in a gentle embrace after the concert. Their laughs and the good mood were evident as they posed for photojournalists, and soon, exclusively for Story, they confirmed they were happy and in love. Though both are trying to keep their private life away from the media, in a relaxed interview in September of that same year, while playing guitar in Morris Studio, where some of the greatest hits emerge, Stjepan entrusted Story's journalist with how much Jelena meant to him and what made him special.
The first anniversary of their relationship was celebrated in Verona, and although both were burdened with business commitments, the beloved Jelena and Stjepan managed to find enough time for each other, so when Stjepan was not on tour, he was a compulsory guest at Jelena's concerts. However, it seems that the frequent separation has made itself felt, so this summer, during which Jelena had many concerts across the Adriatic, and has once again shown why she is one of the most popular musicians in the region, was bad for the relationship. As we learn, Jelen's business plan is still full of business. Already this weekend, she has concerts in Slovenia and Macedonia and is happy to meet with fans in the region.