Program error occurred when processing portfolio hierarchy &

What causes this issue?

The error occurred in the module for generating result data records in
the nodes of the portfolio hierarchy (ISB_PREPARE_RP_GAP).
There may also have been an error in the portfolio hierarchy.

System Response

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

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.