This is caused by a plugin called Via Version. Via version is a plugin that has been added to all our servers that allows players using the newer clients to connect to their older server versions. There is a setting that sets the maximum amount of packets that can be sent and this may be causing the issues.
This setting can be disabled by going to the File Manager tab and going to /plugins/ViaVersion/config.yml
In the config.yml file look for these lines:
max-pps: 800
max-pps-kick-msg: "You are sending too many packets!"
Change max-pps: 800 to max-pps: -1 and that should disable it.
If this doesn't resolve the issue, please send us a support ticket and we can help!