This report concerns the conversion of Vladimir the Great to Orthodox Christianity. Nestor supposedly says that envoys sent to the Muslim Volga Bulgarians reported back that they experienced nothing but “sorrow and a great stench”. Presumably this report is coloured by the fact its writer was a Christian of his time and thus poorly disposed to Muslims in general and ghazis in particular. However, assuming this report has any basis in fact, what might the stench be?
For context, the OP is talking about the Povest vremennykh let (Tale of bygone years), a 12th century Old East Slavic chronicle traditionally ascribed to the monk Nestor.
The Chronicle's overall credibility is consensually called into question by the specialists, and the Baptism of Rus narrative with its "choice between the faiths" in particular is generally acknowledged to be a legend.
The description thus should not be taken to be a literal truth.
As for the interpretation of the passage, this study of negative olfactory descriptions in the Old Russian literature https://cyberleninka.ru/article/v/smrad-velikiy-negativnyy-olfaktoriy-drevnerusskoy-slovesnosti suggests that the meaning is unclear and that it can only be speculated that body odor due to many people, naked feet, lack of incense in the temple etc. was meant.
It would thus seem that we don't really know.
The article goes on to show that usually stench was associated with demons, sin, God's punishment and thus carried a metaphorical subtext. It is not clear if this applies to the chronicle's description, however.