OK I found a solution to the osCsid problem during unsubscribe:
Change newsletter_unsubscribe.php line:
<td align="center" class="main"><br><?php echo '<a href="' . tep_href_link($filename_newsletter_unsubscribe_done, '', 'NONSSL') . '">' . tep_image_button('button_unsubscribe.gif', IMAGE_BUTTON_CONTINUE) . '</a>'; ?></td>
TO:
<td align="center" class="main"><br><?php echo '<a href="' . tep_href_link($filename_newsletter_unsubscribe_done, '', 'NONSSL','') . '">' . tep_image_button('button_unsubscribe.gif', IMAGE_BUTTON_CONTINUE) . '</a>'; ?></td>
this did the trick for me.