Hi, Timo, I think your rose is misidentified. Maybe the plant was budded and the rootstock took over?

The white form of the species should look just like the species in chief - - straight prickles sometimes as long as the leaves, prickly receptacle, and very distinct foliage, with leaflets smaller than a fingertip. Please see Rosa minutifolia for a series of photos of the correct plant, with known provenance from Southern California. Mine came from a botanical garden down there. Forma albiflora is only a white form of the regular species and should not otherwise look different.