What does the full error message say? Also, is this with a eGO server or other servers?
A few possible things you could do (but please answer the above questions first

) -
- Verify integrity of game files (Go to your Steam Library - right click the game - click properties - click the 'Local Files' tab - click 'Verify Integrity of Game Cache'
Go to your DODS Steam folder (For me, it's something like this: C:\Program Files (x86)\Steam\SteamApps\common\Day of Defeat Source\dod, but you can also go to the folder by using the same instructions mentioned above for Verifying game files but under the 'Local Files' tab click 'Browse local files')
- Look for the map you're specifically having a problem with in your map and/or download folder and delete it
- Delete your download folder (as either the map or a sound file relating to it or the server may be corrupt and need to be re-downloaded)
- If it's a custom map or if the server has custom content on it, check your games' multiplayer options to see if you've allowed custom downloads
Feel free to post back if you have any further questions/comments/issues!