Sending a signed transaction to deploy a contract (Ethereum)

Question

My goal is to send a signed transaction to create a smart contract, however, I am facing an issue I have not been able to solve for few days:

When I send the transaction (on a private chain), there are two different ending:

- 1 Web3js tells me it worked, I can see the transaction on the block. However, when trying to contact the contract, I have the following output:

Error: Couldn't decode uint256 from ABI: 0x

When I try to run eth.getCode(contractAddress) in Geth, it returns 0x

- 2 Web3js tells me that the transaction has not been mined for 50 blocks (the node did not have time to mine 50 blocks). But I can see the transaction in the block (in geth).

Here is the code I use:

web3g = result;
getContractInstance(function (error, instance) {
    if (error) {
        console.log(error);
    } else {
        let newContract = instance;
        let deploy = newContract.deploy({
            data: bytecode,
            arguments: [MY ARGS]
        }).encodeABI();
        let gas = web3g.utils.toHex(3000000);
        let gasPrice = web3g.utils.toHex(21000000000);
        let gasLimit = web3g.utils.toHex(4000000);
        let nonce;
        web3g.eth.getTransactionCount(req.body.sender_address)
            .then(function (result) {
                nonce = result;
                nonce = web3g.utils.toHex(nonce);
                let transactionObject = {
                    gas: gas,
                    gasPrice: gasPrice,
                    gasLimit: gasLimit,
                    data: deploy,
                    from: req.body.sender_address,
                    nonce: nonce
                };

                web3g.eth.accounts.signTransaction(transactionObject, req.body.private_key, function (error, signedTx) {
                    if (error) {
                        console.log(error);
                    } else {
                        console.log(signedTx);
                        web3g.eth.sendSignedTransaction(signedTx.rawTransaction)
                            .on('confirmation', function (number, receipt) {
                                if (number == 1) {
                                        // do stuff

Can someone point me out what I am doing wrong?

Note: I am able to send signed transaction using the method above to transfer ether between addresses.

Edit: Estimating gas returns the following: error:

Returned error: gas required exceeds allowance or always failing transaction