Hi @advocatux ,
I ran into the same "signature error", attempting to update my Aquaris 10 FHD tablet. As I don't mind breaking the tablet (as opposed to my phone), I just attempted to upgrade using the development channel, which worked (i.e. no signature error), then once that was running switched back to the stable image, and this time no signature error as well.
Working nicely now, no problems, some bugs I encountered with 15.04 seem to be fixed with that release. Very happy. Will attempt the same procedure on my phone.