Cannot add members to channels from System Console

For feature requests, please see: Contributing Feature Proposals - Mattermost.

For troubleshooting questions, please post in the following format:

From the System Console on the Channels tab, adding members to selected channels fails. Save button clocks and doesn’t return or complete.

Steps to reproduce
Client: 5.0.2
Mattermost Version: 6.1.0
Database Schema Version: 6.1.0
Database: mysql

  1. Go into System Console
  2. Under USER MANAGEMENT click on Channels.
  3. Click a Channel to manage.
  4. Click Add Members button.
  5. After selecting members to Add, click Save button.

Expected behavior
After performing the above steps, I would expect the members to be added successfully.

Observed behavior
The Save button clocks with the message: Saving Config and doesn’t complete.

We have a ticket open for this [MM-40360] P1 - Unable to make changes via Channel Configuration. Endless "Saving Config" state - Mattermost.