[TYPO3-english] Fatal error: Class 'TYPO3\Flow\Package\MetaData' not found

[Svrco Svrco]

Hey everyone,

I'm getting a blank site with

Fatal error: Class 'TYPO3\Flow\Package\MetaData' not found in .../typo3/sysext/core/Resources/PHP/TYPO3.Flow/Classes/TYPO3/Flow/Package/Package.php on line 145

error everytime I install an extension on TYPO3 6.2.9 (PHP 5.5.15, MySQL 5.5.43).

I tried everything but I just can't get it running. The most common solution I found was deleting typo3temp/Cache. I did that several times and even truncated cache sql tables to no avail. I checked and the MetaData class is there.

Weirdest thing is, the site starts working again after a few hours, so it must be some cache or something, but where?

Any ideas please? It's killing me.