So as I reported in the past, I was worried that the aether world is corrupted beyond repair. When I tried to update it with Spigot (the server software we used to run) to 1.16.2, this was confirmed. When I then switched to Paper (another more advanced version of Spigot), it suddenly worked. Now that we have upgraded however, it shows that this was not true. Paper simply ignored some of the errors and other programs (such as the 2D map) still show them.
So I came to the conclusion hat it’s easiest to scrap the aether as announced and look for a replacement as soon as possible.
So, sorry for back-and-forth. I will remove the world in the coming days and then we will see how we can create something new that isn’t broken.