I was just about to point out the known-plaintext scenario of this setup when the server is compromised, but apparently AES is immune to this attack. [1]
I don't believe it. If you have a text of say "WIBBLE" and thats encrypted and thats all you have, then you can run thru every permutation(brute force) and compare until your decrypted text matchs the known text. No shortcuts to this, but still brute/rainbow realms.