SAP Message 60921 - Error during blocking of settlement units for LO &

Diagnosis As soon as a user edits a rental agreement, all of the rentalagreements for the associated rental unit are blocked for other users.This also applies to the data for the settlement units. System Response A system error occurred during the blocking procedure. The data couldnot be blocked (table VIAK02). Procedure Inform your system administrator of the error number.