I was surprised when i got the same error as you and remembered about this topic. after a while, reading the error carefully i realised that ->WARN NPC 31034 class not found
was not the only error in there. in fact the error started at the beggining. What it was, was that the folder that is created inside your gameserver folder from your workspaceDatapack was missing files inside of it.
Call me crazy but what you have to do is.
Open the new folder that's created inside your Gameserver, delete the folder data.
go back to your gameserver directory and copy the data folder from there and paste it to where you deleted the previous data in my case .../workspaceL2jFrozen_DataPack
I hope i made sense and helped you, Also i don't know if this is a permanent solution, maybe its the wrong way of doing things but so far it works