Progress: Using the new firmware, a new Security Policy Server field appeared on the bottom of the RADIUS Setup page, and setting it to a valid IP solved the popup problem - now it says "Current IP has been configured".
But I don't have a security poicy server (not needed for a proof-of-concept config I think), only a freshly installed FreeRADIUS, which still doesn't receive any packets... :/
Please if anyone has ever got FreeRADIUS working with a HP V1910, tell me the secret! :'(