Only one way sync with MS Exchange server in OS X 10.9 Calendar
There clearly was a synchronization error between my Mac with my Exchange account and the Exchange server. Using another exchange account on my mac resulted in normal Calendar syncing.

With the help of the IT department and after making sufficient backups of my mail and calendar itemts, the IT department 'reseted' my account. With a reset everything is deleted from the server, except my username and password.

Now everything works fine again. But I still have not found the cause of this problem.