Well, those guys at winrar are super-efficient.
Got the answer:
Please check if you have Adobe Acrobat XI installed.
We received several reports from users that Adobe Acrobat XI changes permissions of default Windows temporary folder, resulting in incorrect temporary file processing in WinRAR and other applications.
You can see independent confirmations here:
Users confirmed that it helped to manually create another folder like C:\Temp and assign it as a new WinRAR folder for temporary files in WinRAR "Settings/Paths" dialog. Another option is to restore the original permissions of system temporary folder, but Acrobat can corrupt them again later.