Even though my "non-Gigaset" handset, GAP-handset or L410 is presented in the list of internal devices the transfer of contacts will fail, why?

GAP handsets from other manufacturers or the L410 cannot be adressed via the APP. They will be presented in the list of handsets, but the transfer will fail as the APP cannot distinguish them from supported handsets. For many other Gigaset handsets registered as GAP on an IP base the transfer will work.