502 Bad Gateway

We use Elasticsearch 7.8.0. The problem we have is the "502 Bad Gateway" error that appears very often. The problem seems to occur between ms proxy and Kibana, but there is no information in the Kibana logs what could be wrong. If you need additional information, please let me know. I will be very grateful for any help.

Is it something you weren't seeing prior to upgrading? What version did you upgrade from?

Hi Lucas, Thank you for your answer.
We had version 7.4.2 before and the problem wasn't present. However, it did not appear immediately after the upgrade, so I am not 100% sure that the upgrade is guilty.
Today we got rid of ms proxy for testing purposes, but the error keeps showing up.
Any idea what can be wrong?

Connection between users & Kibana is done thru ALB. ALB has target group with just one kibana instance defined. Traffic towards ALB goes thru HTTPS (with legit certificate taken from ACM) and later on traffic is passed using unencrypted connection towards Kibana port 5601. On target group cloudwatch we see 0 responses with 50x error code. On ALB we see a lot of 50x responses and target is marked as fully healthy. From AWS we got information that Kibana might close the connection with TCP RST during the request from ALB and this is the reason of the 50x response. Kibana logs nothing on this (not even in debug messages). Errors are fully random and cannot be simply replicated. Time to get 50x is around 100-200ms so it's for sure not timeout.

What can be the reason to that? How to debug it further?

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