Issue Signature for Composer breaks regularly

[nielsk]

I have regularly the issue that composer breaks and I get the message: "Uncaught PharException: phar \"/usr/lib/plesk-9.0/composer.phar\" has a broken signature in /usr/lib/plesk-9.0/composer.phar:23\nStack trace:\n#0 /usr/lib/plesk-9.0/composer.phar(23): Phar::mapPhar('composer.phar')\n#1" when it is run.

A reinstall fixes it but it shouldn't happen in the first place. Any ideas what could cause the problem or how I can prevent it?