Who is at fault now that BA/AY codeshares between LHR and HEL do not seem to be bookable?

For example, a dummy booking for 18May LHR-HEL in C, returning 22May, shows all BA and AY metal flights just fine and allows the selection of BA metal flights without problems. All AY metal codeshares are nicely shown as alternatives (and there is plenty of availability), but proceeding to select any of the AY metal codeshares results in Error obtaining price quote