Posted originally on Kleis website by Klei Admin "JanH":

The cause of the issue appears to be that the permissions for the C:\Program Files (x86)\Steam\SteamApps\common\Don't Starve Together\mods directory gets messed up somehow and the game is no longer able to fully modify it. (We think it may have been due to a recent change in Windows 10 via a recent update)

Step 1: go to your SteamApps\common\Don't Starve Together directory, open Properties, and on the Security tab click on Advanced.

Step 2: You may need to disable the permissions inheritance on the mods directory. Do this by clicking on “Disable Inheritance”. The button might say "Change permissions", if so, click on that and enter the username and password for your admin account.

Step 3: When prompted click on “Remove all inherited permissions from this object”.

Step 4: On the Advanced Security Settings window click on Add.

Step 5: Then on the Permission Entry window, click on "Select a principal"

Step 6: In the bottom text area, type "Everyone", and then click "Check Names" and then OK.

Step 7: Now that the Principal is set to "Everyone", make sure that "Applies to:" is set to "This folder, subfolders and files", and that "Basic permissions:" has "Full control" checked, then click OK.

Step 8: You should now see the new permission entry for "Everyone" that has "Full control" listed under Access. Make sure you check "Replace all child object permission entries with inheritable permission entries from this object", then click OK.

You can also try simply restarting your PC, if it's been awhile since you've done so.
As well as making sure that you're running the most current version of Windows.

i have tried everything noting will work and yes i did just try this

I have this problem too. I'm on Linux Mint, but it still remains, some of my mods will not download, even after a restart.