Calendar entries not synced from iphone to mac after upgrade to iOS 9 and iTunes 12.3

Before the upgrade syncing my calendar worked fine, both ways between my macbook pro (OS X Mavericks) and my iPhone 5s (iOS 7).

Then I upgraded iTunes to 12.3 and my iPhone directly from iOS 7 to 9.0.2.

After this, when I synchronize calendar, the following happens:

- New entries created on my iPhone are not synchronized from my iPhone to my macbook. -> NOT OK

- New entries created on my macbook are synchronized to my iPhone. -> OK

- Editing existing entries on my macbook creates new entries on my iPhone (creating duplicates instead of changing the existing entries) -> NOT OK

I tried replacing the data on my iPhone with data from my macbook (option in iTunes) -> Did not help

Any ideas what can be done? I am using the calendar a lot, also having many entries (10-15 a day), syncing the past year