How to fix "Steam unable to sync" error?

There are multiple potential solutions to this nuissance, listed in order of complexity:

1. Restart Steam
2. Restart your PC
3. Navigate to your Steam installation folder and delete or rename the file ClientRegistry.blob. That file causes many problems every now and then... While you're at it, you can also try the same with files starting with ssfn - however that may also destroy any family sharing you configured. Either way, you'll have to log in to Steam again
4. Check the game's cache integrity (slow...)

Here's a great way to fix this without nuking Steam. My downloaded Steam Cloud files got out of whack.

In my case, I quit Steam, I found my Steam Cloud save folder on my PC for the game in question, and nuked it. Then I relaunched Steam and relaunched the game. It fetched fresh data from Steam Cloud and then synchronization worked again.

I used SteamDB to find the AppID for my game, then deleted that folder in the following path.

C:\Program Files (x86)\Steam\userdata\[MY STEAM USERID]\[APP ID]

Problem solved!

You might want to try:

1. right click on Torchlight 2 in steam library

2. go to Local files tab and try 'Verify Integrity Of The Game Cache...'

if that doesn't help then probably you still have some remains of the mods that cause this error and you should remove them.