Sprites are two-dimensional images used for viewing far away. Units without sprites are invisible from far away, which is why I find it so very odd that your units disappear when zooming in.
Frankly this is a new one for me. Congratulations!
As an analysis, something is wrong. (Duh). Something with the visual imagery. The game looks through the EDU to find the appropriate model and texture for the distance, and applies that to the screen. Knowing this, Have you made any changes to the EDU that might explain why the imagery is refusing to work?
The campaign map is governed by the map files under world/campaign/etc This is one that is usually NOT edited by players. So a malfunction here is not so easy to find.
Combine the two, and it seems you may have a corrupted file or three. Have you tried re-installing?