[itsme]

@Vibraphon: ich sehe du sprichst deutsch, bleibe aber trotzdem bei Englisch, ist im Forum einfach so üblich. Frag einfch nach, falls etwas unklar ist.

It looks like it's an issue with the path name of the SD card. Android changed both the path name and how they allow / handle write access to it several times. Even worse: additionally it might be different depending on the manufacturer of your device, see e.g.
https://support.anquet.com/support/discu...5000042956
http://www.controlf.net/sd/
If anybody knows a better overview of paths used, please let me know!
I reported that a while ago: http://zubersoft.com/mobilesheets/forum/...p?tid=2978
and it has not got better, just different. Caused by Android, not Mile's fault.
Restore to "Original File Locations" might fail because of not getting write access or because the path of the "Original File Location" cannot be created. This is often the case when you change the device or the OS, e.g. change the Android version or switch from Android to Win10. When the name of the SD card's root folder has changed the original path cannot be created and MSP stores into the MSP folder.

The good side of the way MSP handles it: you can restore a backup and your library will be just working fine.
The disadvantage: this is only the case if you accept that your files are moved wherever it is appropriate.
My advice: when you change device or OS, make sure that restore works correctly and check where your files really are.

You can use both internal memory and SD card with many combinations of paths if you set 
“Let MobileSheets Manage My Files” OFF
“Expose Database File” ON
and take care of synchronizing your song files and folders yourself. You can backup the song files and folders and mobilesheets.db yourself and use reloc.pl from Sciurius' MSPro-Tools https://github.com/sciurius/MSPro-Tools to adapt the song file paths before copying mobilesheets.db to a different device.
pro: maximum flexibility
con: it's absolutely essential that you know what you are doing