&1: Subexpression &2 is not part of the dependent objects

What causes this issue?

The list of dependent objects does not contain the requested expression.

System Response

Each BRF object saves the relationships to the objects used. If this
relationship goes missing, it might be the case that the BRF object is
no longer calculated correctly.

How to fix this error?

Call up transaction BRF.
Call the expression in change mode.
Save the expression.
The system then automatically creates the table of dependent objects
again.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.