While it very likely has something to do with his opposition to the war, claiming:
The incident shows that the Serbian leadership and security services may well take legal action at the request of the Kremlin or Russia’s security services against any Russian citizen whose activism displeases Moscow
isn't much more well reasoned than Serbia claiming:
The paper I was given said that I was banned from entering because I was banned from entering Serbia
We don't know why he was denied entry, if it was at the specific request of the Kremlin or if it was a Serbian border agent too big for his boots. The fact that the reason he was denied entry was so pathetic leans towards the latter, however.