BEFORE DISABLING ONLINE MODE:
Disabling online mode means your server will no longer use Mojang's authentication for Minecraft accounts.
This allows players using cracked Minecraft clients to join your server.
Using these clients, players can choose any username they want. This can result in someone using your username to gain admin permissions on the server.
Additionally, you may experience a reset of player data (inventories, location, achievements, etc.) when switching from online mode to offline mode, since all players will receive a new UUID.
These are the steps to follow:
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Go to your server’s control panel.
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Click on the "Console" section.
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Scroll to the bottom of the page and click the "Disable online mode" button.
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Restart the server.