Internal error: Unexpected EXCEPTION in function module &1

What causes this issue?

An internal program error has occurred. A function module has triggered
an EXCEPTION that the developer had not been expecting or that is not
permitted here.

System Response

The system cannot execute the required function.

How to fix this error?

Make a note of the:
Name of the function module
Transaction
Entries resulting in this error.
Send an error message to SAP, including these details.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.