Using Task Assigning with DDL prevents users from completing tasks

When a user is completing a task, the task is supposed to be deleted from the "task" DB table. Due to a FK constraint from the "PlanningTask" table, the transaction fails (see the attached stack trace).

Root cause:

Caused by: Error : 2292, Position : 0, Sql = delete from Task where id=:1 and OPTLOCK=:2 , OriginalSql = delete from Task where id=? and OPTLOCK=?, Error Msg = ORA-02292: integrity constraint (DBALLO04.FK_PLANNINGTASK_TASK) violated - child record found