I made my little investigation, it seems the Serbian Special Operations Unit was present and could participate at the protest crackdown in Belarus. Serbian soldiers were also seen in Brest. today. 
10 days before the crackdown (July 30) Vucic and Lukashenka had the phone call broadly promoted in state Belarus media. They agreed to meet in person just after the elections.
It's possible that the soldier on the second photo could participate in Sniper Frontier and Polar Star exercises near Brest (and violated quarantine measures). The first photo was taken in Minsk on election day, at one of several checkpoints with the same cars and soldiers.
🚩Official comment of Belarus MOD: Serbian soldiers are in Belarus but they came for drills
