I think your skin have the old comment form field names, you use "author", "email", "url", "comment", while in b2 1.8 the field names are "u", "i", "o", "p", respectively. You hve to change your skin in skins/skinname/_feedback.php where it says:

$Form->text('author', $comment_author, 40, T_('Name'), '', 100, 'bComment' );

and change the first parameter to 'u', for 'author', and similar changes for the other fields.

Anyway, you will find another problem when you solve this. Take a look to [url=http://forums.b2evolution.net/viewtopic.php?t=8867]this thread[/url] to see what I'm talking about

I really don't understand. My admin says Im using version 1.8.1 - but all the skins included in that downoad have the same comment-forms that I've used so far. Downloading other skins from this site is no solution.

Here's my form code:

<form action="<?php echo $htsrv_url ?>comment_post.php" method="post">

			<input type="hidden" name="comment_post_ID" value="<?php echo $Item->ID() ?>" />
			<input type="hidden" name="redirect_to" value="<?php echo regenerate_url() ?>" />

		<table>
			<?php
			if( is_logged_in() )
			{ // User is logged in:
				?>
				<tr valign="top" bgcolor="#eeeeee">
					<td align="right"><strong><?php echo T_('User') ?>:</strong></td>
					<td align="left">
						<strong><?php $current_User->preferred_name()?></strong>
						<?php user_profile_link( ' [', ']', T_('Edit profile') ) ?>
						</td>
				</tr>
				<?php
			}
			else
			{ // User is not loggued in:
				?>
				<tr valign="top" bgcolor="#eeeeee">
					<td align="right"><label for="author"><strong><?php echo T_('Name') ?>:</strong></label></td>
					<td align="left"><input type="text" name="author" id="author" value="<?php echo $comment_author ?>" size="40" tabindex="1" /></td>
				</tr>

				<tr valign="top" bgcolor="#eeeeee">
					<td align="right"><label for="email"><strong><?php echo T_('Email') ?>:</strong></label></td>
					<td align="left"><input type="text" name="email" id="email" value="<?php echo $comment_author_email ?>" size="40" tabindex="2" /><br />
						<small><?php echo T_('Your email address will <strong>not</strong> be displayed on this site.') ?></small>
					</td>
				</tr>

				<tr valign="top" bgcolor="#eeeeee">
					<td align="right"><label for="url"><strong><?php echo T_('Site/Url') ?>:</strong></label></td>
					<td align="left"><input type="text" name="url" id="url" value="<?php echo $comment_author_url ?>" size="40" tabindex="3" /><br />
						<small><?php echo T_('Your URL will be displayed.') ?></small>
					</td>
				</tr>
				<?php
				}
			?>

			<tr valign="top" bgcolor="#eeeeee">
				<td align="right"><label for="comment"><strong><?php echo T_('Comment text') ?>:</strong></label></td>
				<td align="left" width="450"><textarea cols="50" rows="12" name="comment" id="comment" tabindex="4"></textarea><br />
					<small><?php echo T_('Allowed XHTML tags'), ': ', htmlspecialchars(str_replace( '><',', ', $comment_allowed_tags)) ?></small>
				</td>
			</tr>

			<tr valign="top" bgcolor="#eeeeee">
				<td align="right"><strong><?php echo T_('Options') ?>:</strong></td>
				<td align="left">

				<?php if(substr($comments_use_autobr,0,4) == 'opt-') { ?>
					<input type="checkbox" name="comment_autobr" value="1" <?php if($comments_use_autobr == 'opt-out') echo ' checked="checked"' ?> tabindex="6" id="comment_autobr" /> <label for="comment_autobr"><strong><?php echo T_('Auto-BR') ?></strong></label> <small>(<?php echo T_('Line breaks become &lt;br /&gt;') ?>)</small><br />
				<?php }
				if( ! is_logged_in() )
				{ // User is not logged in:
					?>
					<input type="checkbox" name="comment_cookies" value="1" checked="checked" tabindex="7" id="comment_cookies" /> <label for="comment_cookies"><strong><?php echo T_('Remember me') ?></strong></label> <small><?php echo T_('(Set cookies for name, email &amp; url)') ?></small>
					<?php
				} ?>
				</td>
			</tr>

			<tr valign="top" bgcolor="#eeeeee">
				<td colspan="2" align="center">
					<input type="submit" name="submit" value="<?php echo T_('Send comment') ?>" tabindex="8" />
				</td>
			</tr>
		</table>

		</form>

If you have 1.8.1, then in the /skins/ directory you will have a file called _feedback.php. Copy that file to your /skins/skinname directory and then it should work. The code for creating forms and other stuff have changed, and that skin you're using was not updated.