Same issue here running on SLES 11 SP2. I also do not see any errors in log files.
Modified php.ini in /etc/php5/apache2 directory to turn on logging, but nothing reported when upload fails.
Made sure the assets directory is writable by the apache user. But doing a bit of a readup and there is a mention that PHP uploads to a temporary directory before then transferring to final destination. Does SS do this? Where is this temp dir?