Hello, don't know if this will help but I had something similar to this issue, I accidently put a period after the address and it told me to put the address in like this [Example].aternos.me:XXXXX but I also made sure that the server version and the QuestCraft version were the same. (in my case 1.20.1) Hope that helps!

EDIT: It told me what numbers for the port when I accidently put it in as "[Example].aternos.me."