Error occurred while IDoc '&' was being sent

What causes this issue?

The IDoc '&v1&' created was to be sent to an R/2 System.
An error occurred with '&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?

Check the current status of the IDoc. The IDoc may have been sent in
the meantime (repeat send attempt).
If the IDoc has still not been sent properly, please inform your system
administration.

Error message extract from SAP system. Copyright SAP SE.