Only after the patch has been applied and made available to the general public, have Aion community mangers explained that a possible fix for infamous crysystem.dll error during fort sieges has been included in the patch. We have added this information to our 1.5.1 patch notes page and bellow are the details
Optimizations have been made within the game to assist with the performance issues that characters experience during Sieges and other resource intensive activities
This should significantly cut down on the amount of CrySystem.dll errors that players have been reporting
If, by any chance, you are still experiencing crysystem.dll crashes you can let the Aion community mangers know in this topic on Aionsource.com forums.