The Serbian President refused to support sanctions against Russia

Belgrade will not support anti-Russian sanctions, even if this requires the European Union's foreign policy. This was stated by the President of Serbia Tomislav Nikolic at a meeting with Vice President of the United States Joseph Biden.

Background on this from Katehon:

The rather lengthy duration of Biden’s trip (August 15th-17th) indicates that the United States has serious intentions for Serbia. Apparently, Washington has no desire to give Belgrade free rein. Biden will thus hold talks with the President, Prime Minister, and other senior Serbian officials. In addition, a visit is planned to Kosovo to meet with the separatists’ leadership.

Even though the new Serbian government includes more Atlanticists and direct agents of the US State Department, given the American system’s recent failures in governing the world, the White House is increasingly resorting to manual control mode.