When i reload the Scoreboard, or restart the server, this appears in the log:

http://hastebin.com/pegecomace.avrasm

This is just for monitoring your server with VisualVM or Java Mission Control. If you don't use, you can just ignore it.

The reason you get the stacktrace is that your hoster setted a security policy to their Java runtime which prevents to add data to the monitoring interface of Java.

But the Scoreboard isn't working here, that's the only log i get from ScoreboardStats :/

Then you will have another error or it's conflicting with another scoreboard plugin.

I don't have another Scoreboard plugin and i'm using TablistPrefix, would it causing the problem?

Yes enable compatibility mode

It's enabled

And install Protocollib

I have it too

ScoreboardStats runs in passive mode then. You properly have another scoreboard plugin installed. This show up in the sidebar.

MyCommand has a scoreboard feature but i don't use it, could be causing the problem?

Yes that could be. It uses the sidebar too.