As I already got several requests now I think I need to document it here:
Users in Germany do see wrong calendar weeks in Notes 8.5.1 this year. To correct this you need to change a setting in your Notes client. Go to
"File -> Preferences ->Calendar and To Do -> Regional Settings -> Week Numbers"
and change the setting to "Always use ISO-Standard" instead of "Use my operating system regional setting".
Afterwards the week numbers will be correctly displayed according to the German standard.