I think I've figured it out. The fix I put in for the primary: bit of text that was wrongly being added has actually sorted this issue by chance which is why it works on my device.
Looking at your log, I found lines like this:
ExportFragment: uri to copy:content://com.android.externalstorage.documents/tree/primary%3AOpensong%20data%2FOpenSong/document/primary%3AOpensong%20data%2FOpenSong%2FExport%2Ftest__test.osts
01-19 08:57:20.911 10421 13239 13239 D ExportFragment: name:primary:Opensong data/test__test.osts
01-19 08:57:20.911 10421 13239 13239 W System.err: java.io.FileNotFoundException: /data/user/0/com.garethevans.church.opensongtablet/files/export/primary:Opensong data/test__test.osts: open failed: ENOENT (No such file or directory)
The exported file created is stored at: /storage/emulated/0/Opensong/data/OpenSong/Export/test_test.osts
The file name should be getting picked up as test_test.osts, but it is wrongly being identified as primary:Opensong data/test__test.osts
Note that the filename has that primary: bit at the start, but also has the remainder of the file path with the / removed between Opensong data/). The version you have simply removed the OpenSong/Export part from the last part of the path name, which didn't remove the primary: part, but more importantly for you, didn't remove the other folders in the file path. However, my fix instead ignores everything up to the end of OpenSong/Export/ for the name and only uses the last bit of text - i.e. test__test.osts.
This means when it tries to copy this file address using a file provider (to share) it is trying to copy a file that doesn't exist as it is looking for:
/storage/emulated/0/Opensong/data/OpenSong/Export/primary:Opensong data/test__test.osts
This was the same for every file.
For note, when you open the 'Export' option in OpenSong app, it clears the content of the OpenSong/Exports folder ready for a fresh export.
I will push the new version tonight and you can test and let me know.
Thanks for your patience and help in tracking down the issue,
Best wishes,
Gareth