The server considers the protocol used by the client to be outdated. Instead of randomly switching the client to the latest version, find out which version the server accepts.
Step by step
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Look for supported versions in the server announcement. The version label that appears in the server list may not accurately reflect all accepted versions due to proxying.
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Open a separate profile with the version requested by the server from the
Kurulumlarsection in Java Launcher. Also select the same mod installer and deployed modpack version on the modded server. -
If you are managing the server, verify the versions supported by the proxy and backend servers separately. If you're using ViaVersion, check plugin support and logs.
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Log in to the same address as the new profile. If the error text changes, a version mismatch may have occurred. Examine the next error on its own terms.
Keep in mind
Back up your local single-player worlds before upgrading. Unlocking the new world with the old version.