So in the meantime I managed to fix the Echo. Unfortunately it required a second Echo because I needed to dump the image of the main partition. The image can be dumped and written back to the Echo with the diag partition that is also present on the Echo. The image can be transfered via screen and the zmodem. This is rather slow which is why i would recommend compressing the image.
The image also seems to be somewhat region-gated? I tried the English image and it didn't fix the Echo. But a second German image did. In case anybody needs help debricking theirs, I'll be happy to help.