MSIX : problem reading register key in HKCU

aMesquita_ One possibility (assuming the key was placed correctly into the package):

Sometimes older apps, especially with HKCU keys, open the key asking for "Full Permissions", rather than for Read/Write.  This stems from some old sample code Microsoft issued many years ago. The difference in such a request is that permission is granted to delete the key with full permissions.  Chances are the app doesn't intend to delete the key, but that asked for everything. The MSIX runtime blocks such requests.  You can determine if your app is doing with a trace.  

MSIX has no deletion marker capability such as App-V had. I have previously requested that Microsoft consider building a RegistryFixup for the Psf that would allow you to re-mark that request as Read/Write, however there has been no response to that request.