Lambda returns "Malformed Lambda proxy response" - node.js

Question

weirdest thing ever, was trying out one of my endpoints in lambda, everything was working a few days back and nothing was changed.
Basically the functions runs fine up until the point where it needs to return a status code, for some reason, then it just returns a 502 and in the API Gateway it states "Malformed Lambda proxy response"
app.post("/api/v1/documents/create", async (req, res) => {
res.setHeader('Content-Type', 'application/json');
const filename = req.body.filename
const partner = req.body.partner
const payload = req.body
const uid = req.body.uid
console.log(payload)
try {
// Initial setup, create credentials instance.
const credentials = PDFServicesSdk.Credentials
.serviceAccountCredentialsBuilder()
.fromFile("./pdfservices-api-credentials.json")
.build();
// Setup input data for the document merge process.
const jsonString = payload,
jsonDataForMerge = jsonString;
// Create an ExecutionContext using credentials.
const executionContext = PDFServicesSdk.ExecutionContext.create(credentials);
// Create a new DocumentMerge options instance.
const documentMerge = PDFServicesSdk.DocumentMerge,
documentMergeOptions = documentMerge.options,
options = new documentMergeOptions.DocumentMergeOptions(jsonDataForMerge, documentMergeOptions.OutputFormat.PDF);
// Create a new operation instance using the options instance.
const documentMergeOperation = documentMerge.Operation.createNew(options);
// Set operation input document template from a source file.
const input = PDFServicesSdk.FileRef.createFromLocalFile('./darbo_sutartis.docx');
documentMergeOperation.setInput(input);
// Execute the operation and Save the result to the specified location.
documentMergeOperation.execute(executionContext)
.then(result => {
console.log("saving File to TMP?")
result.saveAsFile('/tmp/' + uid + '_' + partner + '.pdf')
const checkTime = 1000;
const timerId = setInterval(() => {
const isExists = fs.existsSync('/tmp/' + uid + '_' + partner + '.pdf', 'utf8')
if (isExists) {
console.log("\nFile written -> creating AWS Bucket")
const params1 = {
Bucket: "darbo-manija-docs",
Key: "employment_documents/" + uid + "/" + partner + "/",
};
s3.putObject(params1, (err, data) => {
if (err) {
console.log(err)
} else {
console.log(data)
}
});
console.log("\nAWS Bucket directory created...")
// do something here
console.log("\nUplaoding file to AWS\n")
fs.readFile('/tmp/' + uid + '_' + partner + '.pdf', function (err, data) {
if (err) throw err;
const pdf = data.toString('base64'); //PDF WORKS
const pdfNew = Buffer.from(pdf, 'base64')
const params = {
Bucket: 'darbo-manija-docs/employment_documents/' + uid + "/" + partner,
Key: uid + '_' + partner + '.pdf', // File name you want to save as in S3
Body: pdfNew, // <---------
ContentType: 'application/pdf'
};
// Uploading files to the bucket
s3.upload(params, function (err, data) {
if (err) {
res.status(400).send(JSON.stringify({
message: "ERR",
code: 0
}));
}
console.log(`\nFile uploaded successfully. ${data.Location}`);
console.log("\nCreating entry in Firebase")
var fb_ref = admin.database().ref('/documents');
fb_ref.push({
documentBucket: params.Bucket,
documentKey: params.Key,
candidate: partner,
employer: uid
})
.then(function (fb_ref) {
admin.database().ref('/documents').child(fb_ref.key).update({
documentID: fb_ref.key
})
});
console.log("\nFirebase entry created");
console.log("\nRemoving temp file...")
fs.unlinkSync('/tmp/' + uid + '_' + partner + '.pdf')
res.status(200).send(JSON.stringify({
result: pdf,
code: 100
}));
});
});
clearInterval(timerId)
}
}, checkTime)
})
.catch(err => {
if (err instanceof PDFServicesSdk.Error.ServiceApiError ||
err instanceof PDFServicesSdk.Error.ServiceUsageError) {
console.log('Exception encountered while executing operation', err);
res.status(400).send(JSON.stringify({
result: "Bad request",
code: 400
}));
} else {
console.log('Exception encountered while executing operation', err);
res.status(400).send(JSON.stringify({
result: "Bad request",
code: 401
}));
}
});
} catch (err) {
console.log('Exception encountered while executing operation', err);
}
});
No idea what is happening, read many posts regarding the same error, but none of them seem to have the same setup. Any suggestions? Thanks