The next two cents: Probably it helps to "allow apps on external" (makes any app eligible to be written to external storage, regardless of manifest value).
Open "Settings", open "Developer options" (if you can't find it open"About phone", tap on "Build number 7 times", go back one time and you should see the additional menu entry), scroll down to the bottom and activate the entry "Force allow apps on external".
Hope that helps, bye for now Fred