Problems with flash upload, strange failsafe error

I have an upload page for uploading multiple mp3 files using uploadify. For me it works perfectly, however one of my users reports intermittent problems, and indeed, errors appear in the logs.

My set up: Rails 2.3.5 + datamapper 0.10.2, latest uploadify, Ruby EE 1.8.7 2009.10, passenger 2.2.9

Here is a successful uploadify post from the user: