unknown upload error

I need help please!!!
I startet a photocontest on http://smartpix.eu/smartcontest/ Unfortunetly an error occurs if you want to upload a picture:
“unknown upload error”

All write permissions are set. My WP is 2.9.2 the plugin is 1.5.6 and the MySql on server is 5.1.

Does annybody know whats going on?

Thanks a lot.

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