Plutonium

@acrylics One topic about the same problem was created 8 hours ago so please check if there is already a topic about your issue before creating one.
This error occurs because of a bad connection or unstable network.
Try mikey's recommendations:

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But, in the mean time, I recommend doing these 3 things:

Restarting router (internet)
Restart computer
Then wait 5-10 minutes, try to join a server. If this doesn't work, wait 1-2 hours and then try to rejoin a server.

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You can also try to have a wired Internet connection.