Debe ser la codificación de caracteres de la tabla phpbb_posts o la de phpbb_topics, o ambas. Mira qué codificación tiene la tabla phpbb_users, porque creo que esa sí está bien, y las dos primeras que dije son las que están mal.
Si eso no soluciona nada quizá estas sentencias SQL ayuden:
Código: Seleccionar todo
UPDATE phpbb_banlist SET ban_reason = replace(ban_reason,'í¶','ö');
UPDATE phpbb_banlist SET ban_give_reason = replace(ban_give_reason,'í¶','ö');
UPDATE phpbb_drafts SET draft_message = replace(draft_message,'í¶','ö');
UPDATE phpbb_extension_groups SET group_name = replace(group_name,'í¶','ö');
UPDATE phpbb_forums SET forum_name = replace(forum_name,'í¶','ö');
UPDATE phpbb_forums SET forum_desc = replace(forum_desc,'í¶','ö');
UPDATE phpbb_forums SET forum_parents = replace(forum_parents,'í¶','ö');
UPDATE phpbb_groups SET group_name = replace(group_name,'í¶','ö');
UPDATE phpbb_groups SET group_desc = replace(group_desc,'í¶','ö');
UPDATE phpbb_lang SET lang_local_name = replace(lang_local_name,'í¶','ö');
UPDATE phpbb_log SET log_data = replace(log_data,'í¶','ö');
UPDATE phpbb_moderator_cache SET username = replace(username,'í¶','ö');
UPDATE phpbb_poll_options SET poll_option_text = replace(poll_option_text,'í¶','ö');
UPDATE phpbb_posts SET post_subject = replace(post_subject,'í¶','ö');
UPDATE phpbb_posts SET post_text = replace(post_text,'í¶','ö');
UPDATE phpbb_privmsgs SET message_text = replace(message_text,'í¶','ö');
UPDATE phpbb_ranks SET rank_title = replace(rank_title,'í¶','ö');
UPDATE phpbb_reports SET report_text = replace(report_text,'í¶','ö');
UPDATE phpbb_reports_reasons SET reason_description = replace(reason_description,'í¶','ö');
UPDATE phpbb_topics SET topic_title = replace(topic_title,'í¶','ö');
UPDATE phpbb_topics SET topic_last_poster_name = replace(topic_last_poster_name,'í¶','ö');
UPDATE phpbb_users SET user_sig = replace(user_sig,'í¶','ö');
UPDATE phpbb_users SET username = replace(username,'í¶','ö');
UPDATE phpbb_users SET username_clean = replace(username_clean,'í¶','ö');
UPDATE phpbb_styles SET style_copyright = replace(style_copyright,'í¶','ö');