I have the same or similar issue as one user above.
Error: Class 'IPS\Theme\Cache\class_forums_front_forums' not found (0)
#0 /home/www/doc/1mysitename/www/system/Theme/Theme.php(926): eval()
#1 /home/www/doc/1mysitename/www/system/Theme/Cache/Template.php(108): IPS\_Theme::overloadHooks('class_forums_fr...', 'IPS\\Theme\\Cache')
#2 /home/www/doc/1mysitename/www/system/Theme/Theme.php(777): IPS\Theme\Cache\_Template->exists()
#3 /home/www/doc/1mysitename/www/applications/forums/modules/front/forums/forums.php(81): IPS\_Theme->getTemplate('forums', 'forums', 'front')
#4 /home/www/doc/1mysitename/www/applications/forums/modules/front/forums/forums.php(38): IPS\forums\modules\front\forums\_forums->_forum(Object(IPS\forums\Forum))
#5 /home/www/doc/1mysitename/www/system/Dispatcher/Controller.php(101): IPS\forums\modules\front\forums\_forums->manage()
#6 /home/www/doc/1mysitename/www/init.php(913) : eval()'d code(20): IPS\Dispatcher\_Controller->execute()
#7 /home/www/doc/1mysitename/www/system/Dispatcher/Dispatcher.php(153): IPS\forums\modules\front\forums\ignoretopics_hook_forumController->execute()
#8 /home/www/doc/1mysitename/www/index.php(13): IPS\_Dispatcher->run()
#9 {main}
Result of this error was a error message on all forums and could be solved by copying the style.