There was at least one serious error during the planning run

What causes this issue?

The other error messages give you information about the cause of the
error. You find them in the application log (transaction
<DS:TRAN.RPMLOG>RPMLOG</>).

System Response

The system has cancelled (reset) all changes, as far as possible.

How to fix this error?

Due to technical reasons, this may not be completely possible for
parallel processing. You should check and remove the cause of error and
re-start the planning run.

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.