So today I finally found it! Clematis elisabethae-carolae is, according to Nicholas Turland (co-author of Flowers of Crete), the rarest flower of Crete. He told me approximately where it was located and this morning, after hours of scrambling around I finally found two specimens. The flowers grow in sinkholes in the northern part of the Ammoutsera valley and have the sweet scent of citrus flowers. Quite a contrast with the extremely harsh landscape surrounding them.