Steps to reproduce:

1. Upgrade from ZBX 7.0.13 -> ZBX 7.4.2
2. Templates -> Select Template Items.
3. Select item -> Attempt to modify item key.
4. Save

Result:

1. Item update hangs on save indefinitely
2. See ZBX Server Log files including patch completion indication.
3. See Apache httpd logs
4. See HAR file.

Notes: The log files and HAR file have been heavily sanitized to remove critical or completely irrelevant information. (Eg: zbx_session, usernames, passwords, cookies, pJSON parsing failures, host up/down, etc.)

Expected:

1. Item saves should complete in the Web UI.

Other Notes

1. This appears to be applicable to any template modification, including linking hosts to templates. This was discovered when I attempted to re-add Zabbix Server to the ZBX Server Health monitoring template which had been updated.