http://forums.b2evolution.net/viewtopic.php?t=6360 can help you out with this issue.

Thank you.
It's a know bug and its fixed allready in the next release.

This we can do in the mean time

change in admin/_item.form.php to patch this:

 
   // ################### COMMENT STATUS ################### 

   if( $Blog->allowcomments == 'never' ) 
   { 
      $Form->hidden( 'post_comments', 'disabled' ); 
   } 
   elseif( $Blog->allowcomments == 'always' ) 
   { 
      $Form->hidden( 'post_comments', 'open' ); 
   } 
   else //if( $Blog->allowcomments == 'post_by_post' ) 
   { 
      $Form->begin_fieldset( T_('Comments') ); 

      ?> 
         <label title="<?php echo T_('Visitors can leave comments on this post.') ?>"><input type="radio" name="post_comments" value="open" class="checkbox" <?php if( $post_comments == 'open' ) echo 'checked="checked"'; ?> /> 
         <?php echo T_('Open') ?></label><br /> 

         <label title="<?php echo T_('Visitors can NOT leave comments on this post.') ?>"><input type="radio" name="post_comments" value="closed" class="checkbox" <?php if( $post_comments == 'closed' ) echo 'checked="checked"'; ?> /> 
         <?php echo T_('Closed') ?></label><br /> 

         <label title="<?php echo T_('Visitors cannot see nor leave comments on this post.') ?>"><input type="radio" name="post_comments" value="disabled" class="checkbox" <?php if( $post_comments == 'disabled' ) echo 'checked="checked"'; ?> /> 
         <?php echo T_('Disabled') ?></label><br /> 
      <?php 

      $Form->end_fieldset(); 
   }