An error has occurred in a generation program

What causes this issue?

When you want to run a report that uses the PSJ logical database, module
pools are generated for runtime optimization and memory optimization.
This module pool cannot be generated since there is an error in the
logical database. This error can only occur, if the SAPDBPSJ program was
changed during the runtime of your program.

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?

Restart the report or transaction.

Error message extract from SAP system. Copyright SAP SE.