The active topics looks like a language file didn't get uploaded. Not sure how it let you go through the upgrade script without it, but the languages/english/activetopics.php language file appears to be missing or empty.

activetopics.php
addevent.php
addfav.php
addfavforum.php
addfavuser.php
addpost.php
adduser.php
add_favorite_user.php
admin
approvepost.php
avatar.php
calendar.php
categories.php
cfrm.php
changebasic.php
changedisplay.php
changemood.php
coppaform.php
deleteevent.php
deletepost.php
doaddevent.php
doeditevent.php
domailthread.php
domovepost.php
domovethread.php
donotifymod.php
dopoll.php
doratethread.php
dorateuser.php
dosearch.php
download.php
do_editprivate.php
editbasic.php
editdisplay.php
editevent.php
editforums.php
editpost.php
editprivate.php
edittopics.php
editusers.php
faq.php
filemanager.php
generic.php
grabnext.php
includepoll.php
includepollresults.php
island_custom_links.php
island_forum_stats.php
island_new_users.php
island_public_calendar.php
island_search.php
island_shoutbox.php
island_top_posters.php
island_whos_online.php
jumper.php
list_address.php
login.php
logout.php
mailthread.php
markallread.php
mess_handler.php
mess_reply.php
modifypost.php
movepost.php
movethread.php
mycookies.php
myhome.php
newpost.php
newreply.php
newuser.php
notifymod.php
online.php
pollmanager.php
portal.php
portal_islands.php
postlist.php
previewsig.php
richtext_text_editor.php
search.php
searchtips.php
searchusers.php
sendmessage.php
sendprivate.php
showaddressbook.php
showday.php
showflat.php
showicons.php
showmembers.php
showprofile.php
smilies.php
standard_text_editor.php
start_page.php
toggleignore.php
verifyemail.php
viewmessage.php
viewmessages.php
viewpoll.php
wrapper.php

Rofl, been dealing with it all day trying to explain to my members why they are having issues. Once they clear their browser they are all happy.