I think it's my PC/laptop. I only have 4GB of Ram and it's still Windows 8.1
But it definitely works better on another PC. It could also be due to the server in general, since the server's RAM limit has been reached. When I download the world and want to upload it to a new server, the maximum is reached. So I think my current server has reached its maximum memory.
Display MoreOptifine is client sided, you don't install it on the server, how could it be the problem?
Switch to Paper 1.20.1.
If you don't use mods(other than Optifine) and are facing the error for more than a few days please follow this article to help Aternos and your ISP identify the problem: https://support.aternos.org/hc/en-us/articles/360027235671
Please read the article carefully and don't skip any steps, questions or suggestions.
Following this article will not instantly fix your problem. It only helps to find the root cause which can then be fixed by the network provider.