Upgrade running: Changes in ACM are locked (decision taken by rule &1)

What causes this issue?

An upgrade is currently ongoing.
In this phase, changes performed by code of Access Control Management
must be locked as they interfere with actions performed by the upgrade.
This error message indicates that you have started an ACM action in the
upgrade phase (initial load, activation of artifacts, object repairs).

System Response

The action has been canceled.

How to fix this error?

Wait for the upgrade to be finished.

Procedure for System Administrators

The detection code uses rules.
The following rule is always executed: When the development environment
is not locked against changes, changes are allowed. This rule allows
cleanup activities when during an upgrade, pre-checks (for example the
search for inactive objects) trigger and the upgrade process temporarily
opens the development environment to resolve these obstacles.
Otherwise, the following reasons exist why changes to the access control
management database tables are currently not allowed:
Rule A: The system is currently in upgrade (and none of the rules below
has triggered).
Rule B: The system is currently in ZDO ("Zero Downtime Option") upgrade,
and the current system is the "Bridge-System".
Rule C: The system is currently in upgrade. The current system is the
original system where no more changes are allowed, while in the shadow
system the upgrade process commences.

Error message extract from SAP system. Copyright SAP SE.