Hi!
502 is usually happening if your varnish is crashed. Run sudo service varnish status and look if it is active. If it failed to start you very likely have a buggy default.vcl file. The one provided by Magento is full of bugs, so you have to debug that yourself. Have you taken the one for Varnish 5.0 from the admin? If yes have you added import std; (magento 2.3.2). In 2.3.3 I have found another bug where on one of the last lines they wrote + s instead of + 300s.
I would start searching this way if I were you.
Best regards
Vincent