This occurs because Moodle runs the "display data" function over the submitted data. If it returns empty, then it assumes that nothing was submitted.

If you have not properly setup an audio or video player and Poodll is handling media playback,  then though Poodll processes and uploads the recording, an empty string is returned instead of a player. 

Moodle will consider the submission empty in this case. And this can lead to the error described

Try to use a simple player to handle mp3 or mp4 at site / course or activity level, and check if the situation improves.

See how to set up MP3 handling with Poodll.