In Java, online-mode controls whether the server verifies the player account online. Keep default validation on a normal Java server directly exposed to the internet.
Step by step
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Draw your connection topology. Does the player go directly into Paper, or go through a proxy like Velocity first?
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Use
online-mode=trueon directly accessed Java server. If there is an account login error, first check the Launcher and verification service. -
If you are using a proxy, authenticate with an external proxy. Match the authentication and forwarding settings of the backservers to the official setup of the proxy and block their direct login.
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First verify on the test server that the player ID, inventory and authorizations are correct. Setting change may affect UUID matching.
Keep in mind
Back up player data and permissions files before changing the verification setting.