This might be it:
Shadow Resolution
VSMs provide significantly increased resolution compared to conventional shadow maps, but shallow light angles (or projective aliasing) and very large local lights can exhaust the available virtual resolution. This can present itself as box-like shadows and bias issues depending on the geometry’s surface. Directional Light clipmaps are much less susceptible to running out of resolution, but very narrow camera fields-of-view can eventually exhaust those as well.
There is no simple solution to solving projective aliasing with shadow maps. Even with VSMs, some care must be taken to avoid the worst cases.