Try using ViaVersion instead, maybe it won't cause this problem. You'll need the following plugins: ViaVersion - Base Plugin ViaBackwards - 1.9+ Support ViaRewind - 1.8-1.8.8 Support
Check your logs when your server starts up instead of joining. You can find them in the File Manager. If you need help reading your logs then feel free to reply to this thread with a paste of them.
You need to download the texture pack, upload it to a file host like Dropbox, and paste in the Share Link that comes with your file upload. Here's a video if you're still struggling
I suppose you could use Skript for that. Maybe something like this?
on death:
if attacker is a player:
if victim is a player
cancel drops
add 1 diamond to attacker
It may be because the server hadn't fully started before they joined, that happens to me sometimes. Make sure the server status on your dashboard is "Online" before joining.