It seems there may be an issue with the order or the process of signing the transaction. Make sure you're correctly building the transaction and then signing the serialized transaction. Here's a sample code that might help:

async function signSerializedTransaction({
  signer,
  serializedTransaction,
}: {
  signer: Keypair;
  serializedTransaction: string;
}) {
  const txBytes = fromB64(serializedTransaction);
  const signatureWithBytes = await signer.signTransaction(txBytes);

  return signatureWithBytes.signature;
}

async function run() {
  const client = new SuiClient({ url: FULLNODE_URL });
  const tx = ...;

  ...

  tx.setSender(USER_KEYPAIR.toSuiAddress());

  tx.setGasOwner(SPONS_KEYPAIR.toSuiAddress());

  const txBytes = await tx.build({ client });
  const txSerialized = toB64(txBytes);

  const userSignature = await signSerializedTransaction({
    signer: USER_KEYPAIR,
    serializedTransaction: txSerialized,
  });


  const sponsorSignature = await signSerializedTransaction({
    signer: SPONS_KEYPAIR,
    serializedTransaction: txSerialized,
  });

 
  const resp = await client.executeTransactionBlock({
    transactionBlock: txBytes,
    signature: [userSignature, sponsorSignature],
    options: {
      showObjectChanges: true,
      showEffects: true,
      showBalanceChanges: true,
    },
  });
  console.log(resp);
}

Make sure that the signatures are properly handled and in the correct order when sending the transaction. The error message indicates that there might be a data formatting issue or a communication gap in how the transaction is serialized and signed.