Cannot assign a person to a task via the dropdown menu or remove an assigned person.

Summary

I cannot assign a person to a task via the dropdown menu in the task assignee field or remove an assigned person.

Assigning over the "assign myself" button works, but using the dropdown menu and delete does not work. As far as I can tell there is no communication between browser and server after the actions in the UI (except when using the assign myself button).

This is on gitab.com and a self-install from source, version 15.4. I see the same behaviour with latest versions of firefox, chrome (linux+windows) and edge, with and without extensions enabled.

Steps to reproduce

Create a task for an issue
Assign someone to the task over the dropdown menu
Reload task page - assignee not visible

Or:

Create a task for an issue
Assign myself to the task
Delete assignee from task (or reassign via dropdown menu)
Reload task page - assignee not deleted/changed

Example Project

See two tasks for issue xis10z/test#1

What is the current bug behavior?

No change of task assignee after reload.

What is the expected correct behavior?

Change of task assignee after reload.

Relevant logs and/or screenshots

Usually nothing in the logs or the browser console. After waiting a long time chrome's console reported:

WebSocket connection to 'wss://gitlab.com/-/cable' failed: WebSocket is closed before the connection is established.
action_cable.js:211 

failed: WebSocket is closed before the connection is established.
t.close	@ action_cable.js:211
t.reopen @ action_cable.js:218
reconnectIfStale @ actioncable_connection_monitor.js:106
(anonymous) @ actioncable_connection_monitor.js:75
r @ helpers.js:72
setTimeout (async)
... (repeated many times)

The websocket connection error is also repeated three times.

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:         Debian n/a
Current User:   git
Using RVM:      no
Ruby Version:   2.7.5p203
Gem Version:    3.3.15
Bundler Version:2.3.15
Rake Version:   13.0.6
Redis Version:  7.0.5
Sidekiq Version:6.4.2
Go Version:     go1.17.10 linux/amd64

GitLab information
Version:        15.4.1
Revision:       b41a72da753
Directory:      /srv/git/gitlab
DB Adapter:     PostgreSQL
DB Version:     14.5
URL:            https://qyber.black
HTTP Clone URL: https://qyber.black/some-group/some-project.git
SSH Clone URL:  git@qyber.black:some-group/some-project.git
Using LDAP:     no
Using Omniauth: yes
Omniauth Providers:

GitLab Shell
Version:        14.10.0
Repository storage paths:
- default:      /home/git/repositories
GitLab Shell path:              /home/git/gitlab-shell

Results of GitLab application Check

Expand for output related to the GitLab application check

System information System: Debian n/a Current User: git Using RVM: no Ruby Version: 2.7.5p203 Gem Version: 3.3.15 Bundler Version:2.3.15 Rake Version: 13.0.6 Redis Version: 7.0.5 Sidekiq Version:6.4.2 Go Version: go1.17.10 linux/amd64

GitLab information Version: 15.4.1 Revision: b41a72da753 Directory: /srv/git/gitlab DB Adapter: PostgreSQL DB Version: 14.5 URL: https://qyber.black HTTP Clone URL: https://qyber.black/some-group/some-project.git SSH Clone URL: git@qyber.black:some-group/some-project.git Using LDAP: no Using Omniauth: yes Omniauth Providers:

GitLab Shell Version: 14.10.0 Repository storage paths:

default: /home/git/repositories GitLab Shell path: /home/git/gitlab-shell

Possible fixes

No idea.