Go to https://aternos.org/worlds/ see you there a world named
{WORLDNAME}_the_end
Yes: can you download and upload it here
No: try to reset your server NOTE: EVERYTHING ON YOUR SERVER WIL REMOVED
Vanilla doesn't have that world system. In vanilla, you can only reach the nether and end via the files in the DIM-1 and DIM1 folders. Only on bukkit servers it shows the dimensions like that