Have a look at your error logs if you can to see why it's failing.
You can also try disabling mod_security if it's running, by adding the following at the top of your .htaccess:
<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>
Or, it might be worth increasing your php memory limit:
1. If you have access to your PHP.ini file, change the line in PHP.ini
If your line shows 32M try 64M:
memory_limit = 64M ; Maximum amount of memory a script may consume (64MB)
2. If you don't have access to PHP.ini try adding this to an .htaccess file:
php_value memory_limit 64M
3. Try adding this line to your wp-config.php file:
Increasing memory allocated to PHP
define('WP_MEMORY_LIMIT', '64M');
Or speak to your host to see if they can shed any light on it (they might not allow you to disable mod_security for instance).
Otherwise, usual troubleshooting applies - disable your plugins and switch to the default theme. Try again. If it works, try and narrow it down by re-enabling them one by one.