Few things about the pack:
the pack you using has alot of problems and backdoors, i recommend you to find a developer to help you remove them
Few things about the error:
its a common NPE in Brasil people have got use to it and ignore them you did some action on this Horus boss and the developer did not predict that in some place the code would be "null" so here you have it, adding a null check will solve this
Few things on how to find that error:
look closely the error says that was generated on
com.l2jmega.gameserver.instancemanager.RaidBossInfoManager.getRaidBossRespawnTime(RaidBossInfoManager.java:78)
this gives you the place and the line to look what happent
respectively RaidBossInfoManager.java:78
so looking at this file on that line is what you have to do, then if you dont know what to do come back here and ask what to do on those lines.
OR
pay a developer to do this for you
@Kara br pack your expertise