Your client may be using a newer protocol than the server accepts. The solution is to select compatible client as player or schedule server upgrade as administrator.
Step by step
-
First check the version announced by the server owner. In Java, you can create a separate Launcher profile for that version without deleting your current installation.
-
In Bedrock, especially console and mobile clients can be updated automatically. Ask the server owner about compatible connections until the server or Geyser version is updated.
-
If you are an administrator, back up the world and plugin data. Try the new jar file on a copy first and upgrade the Java version used if necessary.
-
Login to the test server from current and target clients. Move the upgrade to live if player data and add-ons are also working.
Keep in mind
A version converter plugin does not automatically port new game features to the old world.