When scanning the registry with CCleaner I get a list Of problems ;
it is a list of "uninstall error KBxxxxxx"

looks to me it has to do with the automatic installed Windows updates tagged KBxxxxxx.
Are these "uninstall errors" references only and
Is it save to remove them? Because I do not think of uninstall all of them.

Most likely you did delete the Windows Update Uninstallers (checking the option in Windows > Advanced tab),
so their references in the Windows Registry become unuseful, and worth to be deleted as well. Don't be afraid,
as said many times this does not delete any installed Windows Update, just their uninstallers.