This error has proven to be a tough one to resolve.
This is a custom DoD map. DONE,...at least if it wasn't for this error. I'm not using any custom *.wav's, I detect no problems in the Hammer editor "check for problems" review, only one model (kingtiger), and am not aware of any "ghost entities" magically created by the editor. (it does that sometimes)
I have copied and pasted the entire map into another rmf as well,..but the problem persists.
The map stops during the precaching resources phase in the Steam startup and I get this error: "SZ_Getspace Overflow on netchan <playername> overflowed" Any other mapper had this?,......how did you resolve the issue?
Thanks in advance for any suggestions.

Just an update for anyone interested.
The resolution called for deletion of about 25% of the map area, and re-working to simplify all geometry previously in that area.
Still don't know what was causing the problem as I've had far more complex maps with triple the amount of entities, and they worked fine.
"dod_depot4" will appear somewhere soon.