"There was a problem downloading the audio." This is likely caused by your phone clearing out old files that have not been accessed in a while, a process known as "garbage collection."
Solution 1: Delete the download entry in the Downloads screen by tapping the "Edit" link at the top, then tapping the X for the riff you want to delete, and then trying your download again from the Library screen.
Solution 2: Delete your phone's cached/temporary data and try again.
Solution 3: Reinstall the app and perform the download again.
If none of the above options work, please submit a trouble report through the app's "Report a Problem" feature by tapping the "i" icon, then Contact Us > Report a Problem. The included log file will help us understand the root cause of the problem.