Ok, I found out more. Thwe problem appears to be some compability with the nuvola-icon-design.
Although the nuvola package was not changed, several icons disappered. However, they appeared before! At the moment I changed to the oxygen icon theme, were all icons appear (also the ones of the yahoo weather widget). I do not believe, the problem is in the nuvola package, but something in the libs of plasma 5 might be the cause. As I said, before the last upgrade everything was working fine, and the nuvola-icon-package was untouched. However, it is still a bug and should be fixed. Best Hans