I came across this just now as i encounter same issue but the answers provided helped me to easily know what might be wrong.

Here is what i did to solve it.
I went to my settings to check the ssl of the website. Originally it was http:// but it ought to be https://. All that is needed is to add 's'.
It will log you out- login back and it is done.

In my own case no plugin conflict.
Good luck.