I'll copy paste this from a reddit post I made about this that hasn't gained any traction for context:
So today I made a copy of an Aternos server for beta testing, and I spent some time fixing up the plugins, and I entered the world, everything went fine. Found out some others werent okay, checked the logs, fixed them again,, and now I literally can't log into the server. It just gets stuck on "Connecting to server..." and then I see "Failed to connect to server Timed out", which is an error message that I haven't seen in any thumbnails or photos of any fixes for the error, which 'till a bit ago I didn't notice. I did literally EVERY SINGLE FIX, I restarted my computer 2 times, my Minecraft like 5 times or more, checked my internet (my country has some of the best internet in the world, and I had like 5 ping), removed my anti-viruses, removed the firewall, EVERYTHING, and still nothing worked. I can join every other server, except for this one. I have literally no idea why this happened. It just suddenly became an issue. This is really irritating because I have a lot of fixing and work to do on that server. If I don't get any responses, I'll try making my own fix and update the post for anyone that has the same issue as me, because I feel like this might spiral into a really big and annoying error.
Recently I tried making another Aternos server with the same world, and it still didn't work.