This was haunting me since the release of the first Motorola Moto G and the firmware update of the Nexus 5: The Qualcomm baseband software returns QMI_ERR_ACCESS_DENIED for every request. Recent 64 bit phones seem to all suffer from this. I had no idea what I could do about that.
Until now...
The problem is simply a missing setting in the baseband's NV storage!
This sounds easy to fix but it is not, at least currently. The change has to be made via the Qualcomm diagnostic interface, usually over USB. The first problem is that this port is not available in every firmware, since some manufacturers remove it from the kernel configuration. The probably best known example is the Nexus 5. The next problem is to find out how to switch the USB port on the phone to the Qualcomm diagnostic mode. This seems to be different for every phone and of course every phone needs a different Windows driver which is usually hard to find.
The last problem is that you will need proprietary Qualcomm tools. Luckily the files are available in the Internet. Be cautious not to pick one with malware! I hope I can do the communication with the installer app, but that will need a considerable amount of time.
The good news: I verified functionality with the Nexus 5 and the Moto G (1st gen). As far as I understand the change is stored in the EFS partition, i.e. you have to do the whole procedure only once and it seems to survive flashing a new firmware.
Stay tuned for detailed installation instructions for the Moto G and the Nexus 5. I have two 64 bit phones (ZTE Nubia Z9 Mini and OnePlus Two) which I will try next, I'll let you know the results.