But i can't insert the private key I extracted from the multibit client, can I?
There is a menu with import and export of private keys. After a new installation simply use this to import the previously exported keys.
Ok so this is not client dependant?
Private keys are client-independent. As noted in my previous post, you can import them into a different local wallet client, or into an online wallet. The process varies, but there's always (so far as I've seen) a way to do it.
and what about the blockchain.info wallet? Can I make a wallet there, import the key i got from my multibit client and have the btc show up there? that seems like an easy way to solve the problem..