Robert - I have the same issue - followed this thread down to here, but the url to my portal DOES have https://
Here's the error from dev tools:
I'm not sure where to go next to get this fixed - thanks for any tips!
Edit - looking into it further, I found out that my proxy ping is failing. Trying to troubleshoot that first, but also lost there. I've found tons of help docs and threads that say "try proxy.ashx?ping to see if your proxy is working" but none of them say what to do if the ping fails.
Edit 2 - turns out that the password for the account used to access the arcgis server service had special characters in it that were causing the proxy to fail. I changed the password and removed the special characters and things are now working fine.