Hi Florian,
When any switch connects to the controller, the controller will issue a multi-part request where it asks for the table_features of the switch. For each table that the switch reports, the controller will automatically insert a match=*,goto=NEXT flow at priority=0 to make sure that flows are seen by all tables. The last table is initialized with either output=CONTROLLER or output=NORMAL, depending upon the ControllerManager:hybrid.mode setting.
I believe that mininet reports 256 tables, therefore the controller would push one goto instruction to each of the first 255 tables, then an output=NORMAL instruction to the last table (with the default VAN settings). If mininet is not accepting a goto instruction in each table, then that may be an issue we'd need to raise for the mininet/OVS software (we should be able to program each table).
We have only verified compatibility with OVS 2.3.2, as noted in the VAN application support matrix:
http://h20564.www2.hpe.com/hpsc/doc/public/display?docId=c05040231
You may want to downgrade to that version, unless you need the features of 2.5. If you need features in 2.5, we can try to figure out what's being rejected by mininet. If you take a .pcap on the control-plane when the switch connects to the VAN controller, that would be easiest to diagnose (or you can use "OpenFlow Monitor" in the VAN GUI).
Shaun