Od supportu přišla odpověď:

Hello xxxxxx,

Thank you for contacting Steam Support.

Please follow the link below for more information:

Title: Verifying Game Cache Files
Link: http://support.steampowered.com/kb_article.php?ref =2037-QEUH-3335

After completing this process please restart Steam. This should resolve the issue.

If you have any further questions, please let us know.