Held documents must be converted; read long text

What causes this issue?

The structure of the document data has changed in the meantime. Before
new documents can be held or held documents can be processed, the
documents held up to now must be converted.

How to fix this error?

The held documents are converted by report <ZH>RFTMPBLU</>. For larger
data volumes (more than a couple of hundred held documents), the report
should be scheduled as a batch job.
If only small datasets are to be converted, you can carry out the action
in another window and continue the current processing after completing
the action.

Error message extract from SAP system. Copyright SAP SE.