I have done several upgrade for openSUSE before, so I followed what I had done previously.

After changing the repository URL from ...42.2... to ...42.3...

zypper refresh yield the warning:

File '/repodata/repomd.xml' not found on medium 'http://download.opensuse.org/distribution/leap/42.3/repo/oss/'

After ignoring this warning I run

zypper dup --dry-run

this produced 70 problem. ere are the first three:

Problem: nothing provides libKF5Cddb.so.5()(64bit) needed by k3b-17.04.2-4.1.x86_64

Problem: nothing provides libKF5Gravatar.so.5()(64bit) needed by kmail-17.04.2-3.1.x86_64

Problem: nothing provides libbluray.so.2()(64bit) needed by libavformat56-2.8.13-32.1.x86_64


I reverted to the old /etc/zypp/repos.d

Can somebody help me to find out what goes wrong here?

If you use Yast Software Management you can see those errors more easily and the solutions. I find that the system sometimes gets confused over dependencies when there's a huge update and it looks for permissions to delete some of the old replaceable packages.

I have in the meantime solved the problem myself - with the help of others.

At the heart of the problem was the fact that the repo-oss could not be refreshed
because the file /repodata/repomd.xml was not found.
Once this was resolved everything else worked fine.

To solve the problem with refreshing use the command

zypper refresh -f

If this does not help delete the repository that causes the problem and add it again.

For those who want to know more:

In the directory /etc/zypp/repos.d you find for every repository 'rep' a file 'rep.repo'. In my case that was repo-oss.repo .
In this file the attribute "type" was assigned a wrong value. I have heard explanations how this can happen, but did not understand them.
The refresh command should reprobe and find the correct value of the type. At the moment it does not, which I understand is considered a
bug and may be fixed. More specific it is libzypp.so.1600.15.6 that would need fixing.

But the options -f forces reprobing and thus solves the problem.