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What error message do they get when they try to join?
It might be the IP, their internet connection, or the server is very old. Most often the error messages that you gave happen because the server is either outdated, or maybe you changed something in the server properties.
Is the server old? If it is, you might not be able to play on it. You need to create a new server if you want to play. Go to https://aternos.org/options/ and see if anything is wrong, though.
They get various messages like:
Internal Exception: java.net.SocketException: Connection reset
Timed out
Took too long to log in
Share your server log using the share button on the log page.
For the connection reset error, if it's not modded it's their network issue.
Ask the player who is getting the error to follow this article: https://support.aternos.org/hc…ut-your-server-connection
They are able to join now. In the past it happened again and then randomly they were able to join again. Same thing happened this time, as they can now join the server. The log has reset after they were able to join so there isn't information on that, but I have a screenshot of what was showing up before. In the future if this happens again, what should we do in order to stop it? We don't want to have to wait multiple days until they can join again. 
They are able to join now. In the past it happened again and then randomly they were able to join again. Same thing happened this time, as they can now join the server. The log has reset after they were able to join so there isn't information on that, but I have a screenshot of what was showing up before. In the future if this happens again, what should we do in order to stop it? We don't want to have to wait multiple days until they can join again.
Looks like the server was lagging.
What are the server TPS?
