Problem with minecraft server?

Hi, I'm having problems with my Minecraft server. I made a server for me and my friend but sometimes it randomly doesn't open and gives error for both of us. Other than that, it works fine. I read about it and the people say that it's because another Minecraft is open, but I know for sure it isn't. The only thing that helped was installing another version of MC, but honestly, I'm too tired to do that. I'd appreciate the help
Added (1). we used https://aternos.org
Added (2). Restarting the game does not work

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