**UPDATE**
Steam has "patched" [2] the problem for custom map model files.
Still cant use the custom CAL xhairs or any custom weapon models but at least can still play on custom maps .
Steam Client Update Released
September 22, 2004, 10:56 am ยท valve
An update to the Steam client has just been released. Steam will update itself automatically when you restart. Here is a list of the changes:
STEAM CLIENT CHANGES / ADDITIONS:
* Fixed game launch problem caused by customized game content
Update: In order to expedite the fix for crashing caused by custom content, Steam is temporarily ignoring custom game content. The use of custom content will be re-enabled as soon as possible.
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The problem appears to be related to the latest Steam Update and Custom content
A very quick and dirty fix is as follows:
Delete decals.wad file from dod folder
Delete CAL or any custom crosshair from sprites folder
Delete any custom weapon models.
Note: It has been reported that the v_*.mdl may still work and not p_*.mdl or w_*.mdl.
Rename custom Sprites, Sound and Sounds folders to *_custom.
Make new folders of the same names.
You should now be able to connect to stock maps
Reinstalled dod_railroad , its good (CAL map)
Reinstalled dod_diversion, also good (SLDL week 5)
Forum post on the Steam and DOD forums suggests the issue is limited to custom crosshairs and sounds .
It will take some experimenting to isolate the exact file(s)