Hi Adriano,
I'm not sure if you're still actively supporting/developing this as it looks like IPS have their own version in 4.2, however as I have a pressing need for it, I purchased it anyway and I can report I've having the same problem as Matt above.
I'm running with no other plugins and we're running the stock theme (at least I don't recall changing anything in v4), however, this install has been upgrade over upgrade since the initial release of ip.board so there is many years of crufty config floating around in the db.
The stack trace from the system log is below (which obviously makes me think we have something missing in the theme that you're expecting).
If you need more info or ACP access, don't hesitate to ask.
#0 /home/<redacted>/public_html/mb/system/Theme/Theme.php(780) : eval()'d code(32): IPS\Patterns\_ActiveRecord::load(0)
#1 /home/<redacted>/public_html/mb/system/Theme/SandboxedTemplate.php(57): IPS\Theme\class_core_global_plugins->promoteToArticle(Object(IPS\forums\Topic\Post))
#2 /home/<redacted>/public_html/mb/system/Theme/Theme.php(780) : eval()'d code(812): IPS\Theme\_SandboxedTemplate->__call('promoteToArticl...', Array)
#3 /home/<redacted>/public_html/mb/system/Theme/Theme.php(780) : eval()'d code(812): IPS\Theme\SandboxedTemplate->promoteToArticle(Object(IPS\forums\Topic\Post))
#4 /home/<redacted>/public_html/mb/system/Theme/SandboxedTemplate.php(57): IPS\Theme\class_forums_front_topics->post(Object(IPS\forums\Topic), Object(IPS\forums\Topic\Post), 'topic_comment', 'forums', 'forums', 'forums_Topic')
#5 /home/<redacted>/public_html/mb/system/Theme/Theme.php(780) : eval()'d code(1298): IPS\Theme\_SandboxedTemplate->__call('post', Array)
#6 /home/<redacted>/public_html/mb/system/Theme/Theme.php(780) : eval()'d code(1298): IPS\Theme\SandboxedTemplate->post(Object(IPS\forums\Topic), Object(IPS\forums\Topic\Post), 'topic_comment', 'forums', 'forums', 'forums_Topic')
#7 /home/<redacted>/public_html/mb/system/Theme/SandboxedTemplate.php(57): IPS\Theme\class_forums_front_topics->postContainer(Object(IPS\forums\Topic), Object(IPS\forums\Topic\Post), Array)
#8 /home/<redacted>/public_html/mb/system/Theme/Theme.php(780) : eval()'d code(3226): IPS\Theme\_SandboxedTemplate->__call('postContainer', Array)
#9 /home/<redacted>/public_html/mb/system/Theme/Theme.php(780) : eval()'d code(3226): IPS\Theme\SandboxedTemplate->postContainer(Object(IPS\forums\Topic), Object(IPS\forums\Topic\Post), Array)
#10 /home/<redacted>/public_html/mb/system/Theme/SandboxedTemplate.php(57): IPS\Theme\class_forums_front_topics->topic(Object(IPS\forums\Topic), Array, NULL, Array, NULL, NULL, Array)
#11 /home<redacted>/public_html/mb/applications/forums/modules/front/forums/topic.php(302): IPS\Theme\_SandboxedTemplate->__call('topic', Array)
#12 /home/<redacted>/public_html/mb/applications/forums/modules/front/forums/topic.php(302): IPS\Theme\SandboxedTemplate->topic(Object(IPS\forums\Topic), Array, NULL, Array, NULL, NULL, Array)
#13 /home/<redacted>/public_html/mb/system/Dispatcher/Controller.php(97): IPS\forums\modules\front\forums\_topic->manage()
#14 /home/<redacted>/public_html/mb/system/Content/Controller.php(51): IPS\Dispatcher\_Controller->execute()
#15 /home/<redacted>/public_html/mb/applications/forums/modules/front/forums/topic.php(40): IPS\Content\_Controller->execute()
#16 /home/<redacted>/public_html/mb/system/Dispatcher/Dispatcher.php(129): IPS\forums\modules\front\forums\_topic->execute()
#17 /home/<redacted>public_html/mb/index.php(13): IPS\_Dispatcher->run()
#18 {main}