Calendar / El Capitan / Exchange not working together

After installing El Capitan, I found out that new calendar events with iCal/Calendar are not synchronized anymore: you cannot see them on other devices and event participants get no notifications. Digging a bit into error logs etc. I found this:

07.10.15 12:47:22,021 CalendarAgent[392]: [com.apple.calendar.store.log.persistent.operation] [CalOperationQueue (x-coredata://3909A8BC-D14E-4DAD-AA8C-80A799854ED4/ExchangePrincipal/p21): <CalExchangeCreateCalendarItemOperation: 0x7f9cdd2bed80> failed with error Error Domain=NSURLErrorDomain Code=-1004 "Verbindung zum Server konnte nicht hergestellt werden." UserInfo={NSUnderlyingError=0x7f9cdf1511c0 {Error Domain=kCFErrorDomainCFNetwork Code=-1004 "(null)" UserInfo={_kCFStreamErrorCodeKey=49, _kCFStreamErrorDomainKey=1}}, NSErrorFailingURLStringKey=http://gregor@localhost:0/, NSErrorFailingURLKey=http://gregor@localhost:0/, _kCFStreamErrorDomainKey=1, _kCFStreamErrorCodeKey=49, NSLocalizedDescription=Verbindung zum Server konnte nicht hergestellt werden.}.]

We here have no idea where we can change the connection from localhost to something else. We use the same account info as we do on iPad and there it works perfectly.

Any idea what to do?