I wouldn't say that.
The server is able to boot up and is able to load the world.
As he said that the problem occurs when he tries to join might be a problem with his playerdata (if it has something to do with worlds) or a problem with his plugins
Playing without plugins works. So may it was my plugins. But if it happens again, I should try deleting my playerdata? I did add datapacks to my world but it work for a week or two with them? Thanks (Sorry it was my plugins)