Some more details after a little more exploration:
-sSame phone (Moto e first gen) with CyanogenMod 13
-after a little reading it appears that the api change for sdcard storage requires the app to request card access (see, for example, ESFileExplorer for implementation)
-error message as per attached
-a partial solution - I assume the AM manifest requests emulated/0/backup/AquaMail. If you select this directory (it will need creating) then permissions work and you can restore/backup
cheers
mike