Cause: There are outstanding conflicts logged in the DefError table at the materialized view's master. See the solution below.

Error solution: Resolve the conflicts in the master DefError table and refresh again after the table is empty. Alternatively, refresh with refresh_after_errors set to TRUE, which will proceed with the refresh even if there are conflicts in the master's DefError table. Proceeding despite conflicts can result with an updatable materialized view's changes appearing to be temporarily lost (until a refresh succeeds after the conflicts are resolved).