Hey! Not sure if you still have this issue, I found this thread by looking for a solution myself.
I fixed the "mixed active content" error by switching to UnityWebRequest instead off the WWW class I was using. Seems the WWW thing defaulted to http regardless of what you wrote in the URL. At the same time I forced my server to only accept https, I did these at the same time so I'm not sure if the force https thing is needed. Hope it makes sense :)
(And check out my thing here if you want ;-) https://aergia.itch.io/the-city-haze )