One more thing that I found yesterday, while fixing an issue I had with K9:
When you select a folder to save your attachments to, it loads your file manager/file explorer to locate said folder (or in case you don't have one, you have to enter the route manually). Since I have FX, it said that it didn't recognize any file manager available. I'm gonna try installing another file manager now and see if it recognizes that one.
So yeah, I tried a few file managers and here are the results:
- File Manager (TouchWiz stock)
- File Expert
- ES File Manager
- Astro File Manager
Out of those files, only ES File Manager got recognized by the app. Maybe you could find why is that and try to work yours into K9? Just a thought.