XTU is Intel, not Asus.
And the issue ultimately has to be fixed by Intel, tho the AU update caused it by imposing much more stringent digital signature requirements for all drivers, not just Intels (only on clean installs of AU though, updating to AU from 1511 grandfathers the drivers in. Updating 1511 comes with a different set of problems tho, i think clean install is the better way to get AU even with this issue.) Intel has to get their XTU driver signed like they did for the Intel SSD driver I linked about. It's not a bug, it's MS trying to make secure boot even more secure, now all the driver makers have to register their drivers with MS so they will work. To their credit, Asus appears to have been on top of this change and gotten their drivers signed, I had no problem installing any Asus drivers due to this issue, even the ones released very recently.