Invalid JSON RPC Response – Troubleshooting Invalid JSON RPC in Browser-Solidity

I suspect it is the command you are using.

This works:

user@Kumquat:~$ curl -X POST --data '{"jsonrpc":"2.0","method":"eth_syncing","params":[],"id":1}' http://localhost:8545
{"jsonrpc":"2.0","id":1,"result":{"currentBlock":"0x1eb260","highestBlock":"0x1eb478","knownStates":"0x0","pulledStates":"0x0","startingBlock":"0x1eaea3"}}

And this does not work:

user@Kumquat:~$ curl '{"jsonrpc":"2.0","method":"eth_syncing","params":[],"id":1}' http://localhost:8545
curl: (3) [globbing] nested brace in column 50
{"jsonrpc":"2.0","error":{"code":-32600,"message":"EOF"}}

Here are some simple commands you can test with:

user@Kumquat:~$ curl -X POST --data '{"jsonrpc":"2.0","method":"eth_coinbase","params":[],"id":1}' http://localhost:8545
{"jsonrpc":"2.0","id":1,"result":"0xaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"}

user@Kumquat:~$ curl -X POST --data '{"jsonrpc":"2.0","method":"eth_gasPrice","params":[],"id":1}' http://localhost:8545
{"jsonrpc":"2.0","id":1,"result":"0x4a817c800"}

You need to enable personal API on the command line:

   geth  --rpcapi eth,web3,personal,admin 

More info

However be aware that if you expose RPC socket to public Internet and personal API is exposed anybody can steal your Ether.