Lemmy was released in a new version 0.19.0 which had many bugs and broke federation. Then 0.19.1 was released, but didn't fix all bugs. Now 0.19.2 was released and it works again.
Problem is, that lemmy doesn't allow downgrades to an older version. After feddit.de was upgraded to 0.19.0 and the bugs came into effect, the only way to solve the problems was waiting for the bugfix.