When I try to sign a raw transaction with the same parameters in bitcoind everything works, when I do so via PHP
signrawtransaction('01000000014bdd5...', '[{"txid":"ad0c9...', '["cTbyn7kCBAa7y...')
I get an internal server error from Bitcoin.
Keep in mind that I can easily connect from and issue other json-rpc commands and that the signing works (with each parameter enclosed with ')