Hello. The screenshots should explain it all. The tree has one mesh and there are no gaps between the voxels. If it helps - it was created in Magica Voxel.
Adjust the Bias settings of the light.
No matter what value I put in the light bias, the lines still stay. 
Those lines are from the normal bias, you need to either seal your geometry (no hard edges) or adjust the normal bias on the light, as these push the mesh verts when it’s rendered to the shadow map (hence why having no split verts will fix it).
You can restore the hard edged look in shader or by applying a suitable normal map.
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Thank you for the reply! Yes, I tried to adjust the regular bias earlier, but adjusting the normal bias works for me.