[ DEBUG ]: 09:52:15 | system: Retrieving: dracut-kiwi-lib-9.17.23-lp150.1.1.x86_64.rpm [.error] [ DEBUG ]: 09:52:15 | system: Abort, retry, ignore? [a/r/i/...? shows all options] (a): aSSL error for an http:// URL? Digging for the zypper logs in the chroot found, that this request got redirected to https://provo-mirror.opensuse.org/repositories/... today, which failed due to missing SSL certificates.
[ INFO ]: Processing: [########################################] 100%
[ ERROR ]: 09:52:15 | KiwiInstallPhaseFailed: System package installation failed: Download (curl) error for 'http://download.opensuse.org/repositories/Virtualization:/Appliances:/Builder/openSUSE_Leap_15.0/x86_64/dracut-kiwi-lib-9.17.23-lp150.1.1.x86_64.rpm':
Error code: Curl error 60
Error message: SSL certificate problem: unable to get local issuer certificate
After I had debugged the issue, the solution was quite simple: in the <packages type="bootstrap"> section, just replace ca-certificates with ca-certificates-mozilla and everything works fine again.
Of course, jut building the image in OBS would also have solved this issue, but for debugging and developing, a "native" kiwi build was really necessary today.