I had the same issue than Raúl.
Forcing to use the proxy by changing the QUERY_URL attribute inside initPopupMenu has partially worked for us.
It works when you have only one WMS layer. But if you have two or more, when you click on the "Next feature" icon for the 2nd layer, the Mixed content error appears before initPopupMenu is called.
Do you have any suggestion?
Thanks in advance!