I'm getting an error 500 from my web server when I try to send a raw transaction through the RPC API.

PHP message:

PHP Warning:  fopen(://...@IP:8332/): failed to open stream: HTTP request failed!
HTTP/1.1 500 Internal Server Error
in /usr/share/nginx/html/common/jsonRPCClient.php on line 132

PHP message:

PHP Fatal error:  Uncaught exception 'Exception' with message
'Unable to connect to://bitcoinrpc:PASSWORD@IP:8332/' 
in /usr/share/nginx/html/common/jsonRPCClient.php:140

Stack trace:

#0 /usr/share/nginx/html/tools/do_sendBTC.php(131): 
jsonRPCClient->__call('sendrawtransact...', Array)
#1 /usr/share/nginx/html/tools/do_sendBTC.php(131): 
jsonRPCClient->sendrawtransaction('010000000115d34...')
#2 {main}

This is the line that crashes:

$result = $bitcoin->sendrawtransaction($tx_data);

Every other API call works, for example validateaddress.

Any idea why this fails? The same raw transaction works if sent through the bitcoind command line.

Thanks