Due to problem with our Tebex store XP package on the servers, one of our staff members made a quick test to see what's wrong and why command is not working. Where he played on the server first (so he's in the skill tree data file already) and the executed rcon givexp comman worked properly. But when he left the server and tried to execute the same command again from rcon there's an error
11/08 15:48:46 | [Skill Tree] No player found matching ID: 76561198898188229
So obviously you can't execute any commands such as givexp, givesp, etc. if the player is offline and that's a major problem especially for our Tebext store where players can buy some additional XP and most of the players purchase the package before they're connected to the server.
Please update the plugin to support all RCON commands for offline players as well.