HTTP 504 Gateway Timeout
resolved by setting Connection timeout & Server timeout to 36000ms
(tested with 1000 lines)
after that it worked … export worked like a charm
(now testing if there was a second error within the nginx/next.conf file resulting the content type mismatch)
It’s likely that your server responds with a HTTP 504 error page in the case of a timeout. That causes the content type mismatch as the client expects a JSON response but gets a HTML page instead.