Hi Tobias,
It looks like things are operating now. The '201' status code indicates 'created' and should be considered equivalent to 200 when testing for success. Regrettably the "Response Body" is highlighted in red in our JSON web interface, as shown in your "error_json.png" screenshot:
https://restfulapi.net/http-status-201-created
In general, when you see the REST API return a message that says "Failed to validate flowmod", a good rule of thumb is to enable OpenFlow debug messages on the switch. This will only be useful if the switch is in a controlled environment (because you'd be seeing debug for all OpenFlow operations), but it will tell you why the switch is rejecting the flowmod. In your case above, it would remove the guesswork as to what is wrong with the flowmod (ingress port). If you don't see any openflow debug output on the switch, then the flowmod is being rejected by the VAN controller (but that's a rarer case, usually caused by improperly-formatted JSON).
To enable OpenFlow debug on the switch, use:
debug openflow
debug destination session
Then to turn it off, use:
no debug all
I think the result of the "table-num sw-table-1 0" command would be that the switch would use table 0 in the pipeline as a software table, which means no hardware table would be used. For what it's worth, you should only need to use "table-num ..." commands if you're dealing with a non-VAN controller that doesn't understand the HPE Aruba switch pipeline. In all other cases, just using "pipeline-model standard" (tables 0,100,200) or "pipeline-model custom" (flexible tables, only available on 2930F, 3810, 5400R) should be sufficient.
I hope this helps!
Shaun