The admin vanished unfortunately.

If I remember correctly, he went on some vacation late last year and said he wouldn't have internet, so he gave someone from the community admin permissions. But he didn't give them full access to the server or something like that, so when one day the hard drive had filled up and the instance was quite broken, there was no way to re-install and restore from a backup.

For a long time, the instance didn't allow picture uploads and we just hoped for the admin to return like:

Unfortunately, he didn't. Pretty sure, we still don't know what happened to him.

Well, then feddit.de was completely offline for a few weeks, and when it came back online, the community voted on a new domain name to use and a few folks coordinated on a Matrix server to set it all up.
When feddit.org was all online, moderators of the feddit.de communities put up notices that the community migrated, in case anyone finds out later. Some communities also migrated to https://discuss.tchncs.de.

It was certainly an interesting stress test for this whole federation thing. In theory, everyone could've just joined any other Lemmy instance and in theory, we could've just set up the same communities elsewhere. But in practice, you're hardly going to get all the same people into the same place without being able to coordinate.
It would've helped to spread out the communities beforehand, but that's also easier said than coordinated...