This is just my feedback to let you know.
It was not a iApp issue on the BigIP. I used "simple monitoring" for the exchange iApp which includes all the imap configuration if enabled as well. I did not know that the specific CAS servers need to be configured correctly in this case. But RTFM is all about
This help step is written on the buttom of the form if imap is enalbed
"You must enable IMAP4 on your Client Access Servers before the service will be available.
You must configure the Authentication properties for IMAP4 on each of your Exchange Client Access Servers to allow logins using plain text."
So, it was just a CAS server konfguration and it works now!