That was quite a while ago…
Make sure you try the following in this order:
-
Allow the app to ‘Access Media & Photos’ in Privacy Guard (relatively obvious)
-
Under Settings->Developer options enable Force allow apps on external
(if you do not see Developer options, tap 7 times on the Build number in Settings->About phone (This will hopefully do the trick) -
Change a few lines in
/system/etc/permissions/platform.xml
according to this post: SD Card no writing permission in FP2
This used to work in FPOOS 5.1 but I think it is deprecated now. (Probably neither necessary nor helpful)
I do not remember, what I did exactly, but whatever I did survived any system upgrade, so you only need to do it once