It was not possible to send any IDoc

What causes this issue?

The first IDoc in the selected queue has incorrect syntax.
Since you do not wish to send any IDocs with syntax errors, the queue
will remain unchanged.

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?

Consider what you want to do with the IDocs with incorrect syntax (send
anyway, set to error status, remove from queue).

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.