I've scoured across the internet to find some possible fixes that might work for you (Although you can do the same thing by simply typing "An error occurred while attempting to download a file from the UGC server!" in your browser)

Yep, this error is like a cancer. Either there's a cure for it or just a temporary treatment. It sucks we know.



But before we do such procedures, I have some questions first.
- Have you tried downloading large files from any of your browser? Does it work? Did it stop due to connection error?
- Does this only occur in games like L4D2, Gmod, etc.?
- Any problems with your SSD/HDD? (Running out of space, Bad sectors, etc.)

It might not work but there's no harm giving it a shot

- Change download region
- Clear download cache Clear download cache Clear download cache
- Clean/delete temp files from your drive
- Try Ethernet cable if you're using wireless adapter
- Allow steam in the firewall
- Reinstall Steam (Speaking of harm, backup some of your games / move your steamapps folder away form the Steam folder. Put them back once you're done reinstalling)

1st Fix

- Clear Download Cache from Steam
- Exit Steam
- Delete L4D2 Userdata files. Head to Steam\userdata and find the folder associated with L4D2
(If you don't know which folder, just open the steam_appid.txt form your L4D2 folder)
(Folders in userdata are based on app ID. Find the right one and delete it)
- Delete another folder located at Steam\userdata\###\ugc\dowload
- Move all of your mods into a new folder
- Launch Steam
- Launch Left 4 Dead 2
- Wait for your mod to download
(Once your mods finished downloading, try to download another mod to see if the error still exists)

2nd Fix (This one is a bit complicated)

- Run the Command Prompt as admin
- Type the following command
1. netsh int ip reset (Press Enter)
2. netsh advfirewall reset (Press Enter)
3. netsh winsock reset (Press Enter)
4. ipconfig / flushdns
5. ipconfig / release
4. ipconfig / renew
- Once it's done restart your computer and see if it works

3rd Fix (If and only if you can't download anything from your Computer)

- Run System File Checker and Deployment Image Servicing and Management commands
1. Press the Windows key, type in CMD right-click and click on "Run as an administrator".
2. Copy-paste: sfc /scannow
3. Press Enter. Wait until it completes.
4. Copy-paste: DISM /Online /Cleanup-Image /RestoreHealth
5. Press enter. Wait until it completes.

Sauce #1[answers.microsoft.com]
Sauce #2[answers.microsoft.com]
Sauce #3[answers.microsoft.com]

I might add another fix if all of those methods didn't work