I just realized my mistake. I assumed that importing a cal file would automatically apply it. In reality it only adds the file to the list of available files, and at that stage the app does not check the format or extension, it imports any file.
To actually apply the calibration file, I needed to use the Load Cal option. When I do that with the UMIK-1 files it works, or at least no error is returned. When I try files in unsupported formats (e.g. a JPEG) it fails and returns the error “Cannot read from Storage.”
I probably should have RTFM. Sorry for wasting everyone’s time, but hopefully this thread will help other newcomers as well.