Comment # 3 on bug 1232565 from Stanislav Brabec

Using --debug, I found a source of the problem. It stopped to work when I
created ~/.ssh/id_ed25519 and ~/.ssh/id_ed25519.pub.

DEBUG: Using ssh key from file: /home/sbrabec/.ssh/id_ed25519
Server returned an error: HTTP Error 401: Unauthorized

If I delete these files, build.suse.de starts to work again.

With a fresh home directory, the work-around does not work:
DEBUG: Searching ssh keys in '/home/test/.ssh' in the following order:

id_ed25519, id_ed25519_sk, id_rsa, id_ecdsa, id_ecdsa_sk, id_dsa
Server returned an error: HTTP Error 401: Unauthorized

Surprisingly, once it starts to work, I can return these files to its place,
and it still works. (Maybe up to the next reboot or re-login.)