EDI: Field DOCNUM in data record is invalid

What causes this issue?

An error was identified in a data record while adding an
<DS:GLOS.IDoc>IDoc</>.
Field DOCNUM contains the value &v1&, which is not permitted.
The incorrect value appeared first in segment &v2&.

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?

Please inform your system administrator.

Error message extract from SAP system. Copyright SAP SE.