Well, that can happen when you try to start the server when there are too many users trying to start their server and the Aternos' machines are overloaded (usually in the afternoon). Btw, what modpack have you tried to use? Does the same occur if you reinstall the server? Have you tried to start it in a different moment?
So I tried World of dragons a couple times, didn't work. So I downloaded enigmatica 6. I tried it once, it crashed. Then I tried it again, it worked. Then the next day (yesterday) I tried it multiple times throughout the day, it didn't work. Next day, (today) I then redownloaded it (recommended to me in another post) and tried it again, didn't work. I stated that in the post, and another helper suggested I just attempt to play on a different pack. Before I downloaded a new pack, I wanted to make sure and see if there was any possibility to get it working. Nope. I tried all day, crashed every time.
Also, don't get me wrong, this isn't a complaint about the servers. I completely understand that bigger modpacks might have difficulties loading up and also the machines being completely overloaded. I simply want to suggest, for the good of users (and helpers who get complaints about crashing), that they clear out the modpacks that don't work.
