net::ERR_EMPTY_RESPONSE: only on deployed version - reactjs

Question

I get a net::ERR_EMPTY_RESPONSE error:
only on deployed version, not localhost
other GET-requests work
It downloads all my files from the ftp-server but it returns an error.:(
Flask Backend:
app = Flask(__name__)
cors = CORS(app)
#app.route('/downloadftp', methods=['POST'])
#cross_origin()
def download_all_ftp_data():
# connect to sever...
# download files...
for f in ftp.nlst():
fhandle = open(f, 'wb')
ftp.retrbinary('RETR ' + f, fhandle.write)
ftp.quit()
return 'OK 200'
React Frontend:
useEffect(() => {
axios
.post(`${process.env.REACT_APP_HOST}/downloadftp`, { content: "post" })
.then(res => {
console.log(res)
setError(false)
})
.catch(function (err) {
console.log(err)
setError(true)
})
}, [])

Answer 1

The error is due to the amount of data to download as it is described here: getting this net::ERR_EMPTY_RESPONSE error in browser
The solution for my problems was creating a daemon thread. The response is sent directly after starting the background thread.
from multiprocessing import Process
#app.route('/downloadftp')
#cross_origin()
def get_all_ftp_data():
"""creates a daemon thread for downloading all files while the response it sent directly after starting the background thread."""
daemon_process = Process(
target=download_data_multiprocessing,
daemon=True
)
daemon_process.start()
return Response(status=200)
def download_data_multiprocessing():
"""triggers download current data from ftp server"""
# connect to sever
# download
for f in ftp.nlst():
if (f != '.' and f != '..' and "meteo" not in f):
fhandle = open(f, 'wb')
ftp.retrbinary('RETR ' + f, fhandle.write)
ftp.quit()