Some questions:

• Does it work if you post it somewhere else?
• Does it work if you post it to the same computer?
• Is it some kind if upload or other that demands that you use a special enctype such as "multipart/form-data"?
• Silly question, but: your form is correct? What happens if you post something else to the same place?
• And what happens if you post something from someplace else to the same place?

As you can see, I am trying to isolate the problem, and create a rudimentary "test suite". This is how I usually track strange problems. :)