Minecraft Aternos server not found?

I made a server on Aternos and a friend of mine wanted to join it but it didn't work. The other 15 people or so it worked. We did screen transfer and it had the right IP and version. It just says "Server was not found" and "Error" in the server bar. He restarted MC, updated it and waited a day but nothing helped. Why is that exactly the case with him and how can you fix it? Please be important!

Is the server address correct?

Is the Minecraft version correct?

was the server online when it tried to add it?

Is a whitelist activated?

Yes, yes, no, no. I said yes, the ip and version were correct and I didn't have a whitelist, otherwise the others couldn't figure it out