Thanks for the hint. I found the “repodata” directory in a subdirectory “suse” below “…/oss” respectively “…/non-oss”. Well, I did not change my repository configuration since installing Leap 42.3, so I suppose there was an adjustment in the directory structure of those repositories. Strange that I seem to be the only one to notice that.
Anyway Yast does not complain any more, so I hope refreshing works properly.