Open the app and choose “Settings“. Click on “Memory location“ and choose “External storage“. Now your content is stored on the SD card.
Since Android 4.4 (KitKat), Google doesn’t permit to choose any file on the external SD card for storage. There’s rather a special memory location that is reserved just for this app. Therfore, you should choose the preset list “External storage“, which calculates the automatically reserved memory location.
If there are problems with the storage on SD card, there might be an error that is related with the operating system and affects particular devices. There are already a few tickets on this behalf, and we’re confident to be able to resolve such errors in future.
(cfl. Issue 81357 and Issue 191379).