Hi Heiko
Yes. It's is possible.
But:
1. Such topology is not supported by Cisco - as FEX should be used for connecting hosts.
2. FEX doesn't support local switching which means that all traffic between 2 FEX ports will be sent to the master switch. And the traffic comming on the port where switch will be connected will only increase the oversubscription of the fabric links and introduce latency and traffic loss.
3. By default FEX will block the port because it has BPDU guard configured by default to avoid switch connection.
HTH,
Alex
