Data record (variable key) missing (internal error)

What causes this issue?

The system was unable to read a data record (variable key) for the
(internal) condition record number &V1& from an internal table.
The error occurred in function module &V3&. The condition record
belongs to rebate arrangement &V2& (arrangement number may be missing).
The problem may be due to one of the following reasons:
faulty call of function module &V3& with invalid or inconsistent data
(program error)
database error (that is, the variable key is missing from the relevant
database table.

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?

Contact your system administrator.

Error message extract from SAP system. Copyright SAP SE.