Here are a few conflicts between the clergy and the arcanists that could arise based on the domains and aspects of the deities you've listed:
Bilobog vs. Horos: Bilobog, the God of Light, is associated with healing, protection, and luck, while Horos, the God of Mystery, is associated with magic and knowledge. These two deities may come into conflict if the clergy of Bilobog sees the arcanists as meddling with forces beyond their understanding, while the arcanists view the clergy as holding back progress in the realm of magic.
Chornobog vs. Kupala: Chornobog, the God of Darkness, is associated with destruction and trickery, while Kupala, Mistress of the Waters, is associated with water and decay. The clergy of Kupala may view the arcanists as practitioners of dark and dangerous magic, while the arcanists view the clergy as hindering their research and experimentation.
Svarog vs. Dazhbog: Svarog, the God of the Craft, is associated with creation and knowledge, while Dazhbog, the Giving God, is associated with the sun and healing. The clergy of Dazhbog may view the arcanists as delving into the realm of creation without proper reverence for the gods, while the arcanists view the clergy as holding back their understanding of the universe.
Mokosh vs. Veles: Mokosh, the Goddess of Nature, is associated with healing and magic, while Veles, the Shapeshifter, is associated with trickery and change. The clergy of Mokosh may view the arcanists as not respecting the natural order of the world, while the arcanists view the clergy as not understanding the complexities of the universe and the need for change.
These are just a few examples, and there could be many more potential conflicts between the clergy and the arcanists based on the different domains and aspects of the deities in your pantheon.