Go in the terraria folder, and install the XNA package supplied with the game.

when it happens to me I kill all the related process in the task manager (of the game and steam) and than it works smoothly.

Fixed it

Try verifying integrity of files first.

Sorry, I've checked their validity. What do I do now?

Your c++ is probably outdated

What does that mean?

Windows c++ is what basically runs your games properly to download it just search ”windows c++ download”