discussions.json endpoint in MR returns 502 error

Summary

Comments/discussions in MR fail to load because discussions.json endpoint in MR returns 502 error. Increasing unicorn timeout loads them but shouldn't be necessary.

Steps to reproduce

Create a MR with comments/discussions like follows:

What is the current bug behavior?

Comments/discussions fail to load due to discussions.json returning 502 error

What is the expected correct behavior?

Comments/discussions load

Relevant logs and/or screenshots

Please advise which specific logs you're interested in. I can't provide them all as I have to sanitize them.

Last reproduced on GitLab 11.10.4.

/cc @jramsay as you asked me to file new issue at https://gitlab.com/gitlab-org/gitlab-ce/issues/42493#note_141437843