DDE connection to Excel has been interrupted (error code &)

What causes this issue?

Only after you have permanently saved a report, can you send it as a
mail.

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?

When you execute a report, check of the indicator 'Permanently save
data' on the selection screen. After you do this, you can send the
report as a mail.
Technical background: If you send the report as a mail, only a reference
to the saved data is sent out. If the person receiving the mail edits
it, the corresponding report with the saved data is automatically
called up. All of the Report Writer functions are then available for
use.

Error message extract from SAP system. Copyright SAP SE.