Access Ops Manager web UI failed with "502 Bad Gateway" post restart of tempest-web service

Products

• Operations Manager

Issue/Introduction

Sometimes tempest-web service on Ops Manager server needs to be restarted to fix some issue. However it's been found occasionally accessing Ops Manager web UI would return "502 Bad Gateway" after the service is restarted with command "service tempest-web restart". 
Following error message could be seen in /var/log/opsmanager/error.log

2020/08/11 12:42:53 [crit] 1059#1059: *245039 connect() to unix:/tmp/tempest-web.sock failed (2: No such file or directory) while connecting to upstream, client: 10.0.0.2, server: , request: "GET / HTTP/1.1", upstream: "http://unix:/tmp/tempest-web.sock:/", host: "opsmgr-03.example.io"`/tmp/tempest-web.sock

And file /tmp/tempest-web.sock doesn't exist on the server.

$ ls -l /tmp/tempest-web.sock
ls: cannot access '/tmp/tempest-web.sock': No such file or directory

Environment

Product Version: 2.8

Resolution

This issue has not been observed in Ops Manager v2.9 or higher. We recommend customers upgrade to LTS version of Ops Manger v2.10. Following workarounds are available to solve the issue in case it happens. 

1) Upgrade Ops Manager to v2.9+ if possible
2) Or try to restart tempest-web service for several more times, which will bring it back to normal state eventually