Maybe...
Android is a pain! You tell opensongapp where you want your opensong folder - because this makes Android add a folder that remains even if opensongapp is uninstalled (good).
BUT... Android increasingly 'helps' users to add or use folders in ways that work but will not work after you turn your phone off and on again! Lots of strange things happen as opensongapp cannot update the folder!
Might help to use the advice related to selecting a folder to select your existing folder again and see if that helps.
I use internal storage as recommended - if it fails then the device stops working so you have bigger problems.
It may not be your issue - it is a good thing to check folder selection when strange things are happening.
Sarah