When you click on this Link: http://www.phpBBXS.eu/search.php?search_id=newposts (Newest Posts from your last visit) there's the possibility to see the posts not read and to select those to mark like already read.
Well: If u check 1, 2, etc. posts and click on button "Mark Read", the page redirect to Search: http://www.phpBBXS.eu/search.php
I think the bug is around line #55, a this part code:
<td class="catBottom" colspan="8">
<input type="submit" class="mainoption" value="{L_SUBMIT_MARK_READ}" />
<input name="mar" type="hidden" value="1" />
</td>
<input type="submit" class="mainoption" value="{L_SUBMIT_MARK_READ}" />
<input name="mar" type="hidden" value="1" />
</td>
cYa All.