"If the number of Pages exceeds Max Pages corruption will occur, which sometimes manifests visually as a checkerboard pattern, or corrupt or missing shadows. If the shadow page pool overflows, a warning is displayed on screen and in the log.
If this occurs, increase the size of the page pool or decrease the shadow resolution or number of virtual shadow map casting lights."
As for the disappearing shadow, try enabling nanite on the meshes. Non nanite meshes will not draw into the shadow maps at unlimited distance, and will either not cast shadows or revert to a technique such as distance field shadows beyond a certain range. There is more info in the same page. I’d recommend reading it entirely.