No entries made in reference field

What causes this issue?

When calling up the update interface of financial accounting, an entry
must be made in the reference field "Application key" so that the
document from the calling application can be found again.

System Response

System error

How to fix this error?

There is a program error. Inform your SAP consultant or get in touch
with SAP directly.

Error message extract from SAP system. Copyright SAP SE.