Diagnosis
Using the delayed print function, payment forms were already printedusing transaction FPRD. The system assumes that all installments areincluded in the payment form that have a due date up until the date thepayment form is printed. The system therefore locks these installmentsto changes in the installment plan.
If you now change the due date of a given installment (for which it maybe that no payment form was printed), then the system locks thatinstallment to further changes, as long as the new due date is not afterthe date of the payment form print run. Check if this is really what youwant to do.