Input error with char. &1, value &2; cause: &3

What causes this issue?

An error occurred processing external input data for characteristic
values.
The error occurred in:
Characteristic: &v1&
Value: &v2&
Cause: &v3&
The cause is shown as an EXCEPTION.

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?

In background processing, this message always causes the process to
terminate.
In IDoc processing, the IDoc can also be processed in dialog. This
means you can find out more information on the error and solve the
error in dialog.
You may be able to resolve some error messages for characteristic
values directly from the error message.
Example: external value is written incorrectly.
Other errors can be solved in dialog once all input data has been
loaded.
Example: characteristic name is written incorrectly, so no value is
assigned.

Error message extract from SAP system. Copyright SAP SE.