I used to have these errors in L2jServer freya client some years ago. Monsters have skills too. There are monsters which have skill id 6152 in their skill list but this skill doesn't exist in your server. In order to fix this error, you could either add skills with these Ids (something i don't recommend) or delete these skills from these monsters.
Your pack either uses XML files for skills/npcs or an sql table. Mostly sql table, i think. So, go find them in database and delete the queries with skillId 6152 and etc.