The Balkans too, just with somewhat lower criticality, because the quantity of huge rivers is incomparable...Situation is dire for all Europe really. We're hit bad by the drought and we're currently in the 4th heat wave since May but it's also the case in Italy, Germany or Spain. I don't know about Eastern Europe but they are probably suffering the same.
Serbia has not only Danube, but two long rivers Tisza and Sava (two rivers longer than 900km receiving water from multiple countries), it is among the countries with most water considering the population of only 8 million people.
The drought still impacts the industry and energy production though.