In ePO 4.2.0-rc.1 the cardinality of predicate epo-ful:refersToOrderLine was changed from [0..1] to [1].
Since ePO 4.2.0 is a minor version, such cardinality changes are not allowed. Consequently, epo-ful:refersToOrderLine shall be reverted back to [0..1] for ePO 4.2.0.