I tried a couple test comments. If you look at the error message, you'll see the number "500." This is a generic error message (along the lines of the "404" page/file/resource not found error.
A "500" error usually involves some sort of server misconfiguration. For Apache, these settings are saved in various config files.
if you leave a comment it gives you an error once you submit it. However, we are still able to publish it.
Since the actual posting of the comments is okay, then the issue must be related to something that WP is doing after that, namely, the redirect back to the post.
I'm thinking that this involves the permalink settings. Your site is not using pretty permalinks currently, but if you were, then your htaccess file with the rewrite instructions would likely still be in the root of your website.
Anyway, check to see if there is an htaccess file in your root and see what contents, if any, it has. If you have something close to the following, then at one time, pretty permalinks were set.
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
In this case, go into WP and apply the pretty permalink settings. Then, try commenting again. If you get redirected back to the post, the problem should be solved.
If you're still having problems, please detail your permalink settings and the contents of your htaccess file, if it exists.