Look at the error log, its in the "log" directory.
You can also turn up the debugging/logging level by changing this
// system error reporting: 1 = log; 2 = report (not implemented yet), 4 = show, 8 = trace
$rcmail_config['debug_level'] = 1;
in the config/main.inc.php file.
This will give you a little more information about the problem