I had the same issue, I basically re-installed diva and that corrected the problem for me.
Oconv basically packages your geometry into nested voxels for use in Radiance simulations. This error is usually due to one of three things:
1. Way too many triangles in the scene.
2. Overlapping surfaces.
3. An area of your file that has many extremely small triangles.
Usually Rhino's meshing algorithms don't produce something that could trigger this. Could you share the file?