Unexpected error when installing/uninstalling add-ons in JIRA Data Center undergoing active (ZDU) upgrade

Summary

When JIRA Data Center Cluster is in upgrade mode (i.e. ZDU), trying to install or uninstall any add-ons will fail with the following error:

An error occurred during installation. Refer to the logs for more information.

Environment

JIRA Data Center in upgrade mode (ZDU)

Steps to Reproduce

1. Put JIRA Data Center in upgrade mode
2. Try to install or uninstall any add-ons

Expected Results

JIRA should either give a warning that installing / uninstalling add-ons in upgrade mode is not allowed, or should actually allow to perform the operation

JIRA should also report the results of the operation in the logs

Actual Results

the following error is displayed:

An error occurred during installation. Refer to the logs for more information.

On checking the logs – no information about the operation attempted is registered as well.

Workaround

Complete or cancel the upgrade and install the add-on