Did you verify your game cache?

I did, but the problem persist

Serious Sam 3 Log says this:

20:02:08 LOG: Error occurred while reading cloud package Temp/Workshop/41070/Subscribed/zz09c10c8296f42f00.gro; package will be redownloaded on next application start...
20:02:08 FTL: Cannot load 'Content/SeriousSam3/Levels/Menu/Intro.wld': Required license isn't available. (Content/SeriousSam3/Models/Weapons/Explosive/C4_FP.mdl)

I guess it's a Steam problem, damn.

Have you tried redownloading the mod?

I did, but i'm still getting the same error.

This is the problem:
http://forums.seriouszone.com/showthread.php?t=64586

Do you have the right version of the game installed, Chipson? There's the regular version and the "moddable" version. Sounds like the license error is referring to incompatability with non-moddable version.

Yep, the game installs both versions anyway. I solved the problem by installing the DLC mission, just like Ryason's link says.

So no fix for people without the DLC? sad :c

Well, Serious Sam series are usually very low on Sales. Around 90%, i bought the DLC for $0.99 LOL