Union: Association & is different in the selections

What causes this issue?

If associations are to be exposed in a CDS view with UNION or UNION ALL,
the respective associations must have identical characteristics in all
selection lists.
This means that the following characteristics of assocations must be
identical:
Names or alias names
Specifications relating to cardinality
Association Targets
ON conditions
Position in the Selection List
In this assoziation &V1&, a difference was found in the item &V2&.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

System Response

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.