I think it is the recording size in 1 case, which is 5-6 minutes. In another assignment the instructor has set it to both file upload and online PoodLL - .mp3. I seem to recall that in the past that caused problems.
Permissions are OK.
Small recordings are accepted.
Max upload size for course is 400MB, same for the assignments I checked.
No suhosin enabled.
I'll test the last suggestion when I can, but since this isn't occurring in an on campus lab, any error I can create here may not have the same cause as the students, who are usually on a wireless, off-campus.