I have also been experiencing this issue, but was able to workaround it, as follows. (I never said it was elegant, but it got the job done for me!!)
- Navigate to the point-to-site configuration page (the one featuring the greyed out button)
- Press F12 to access developer tools
- Add a css rule like: .fxc-section-wrapper { display: block!important; }
- This will spew a load of mess onto the form... BUT crucially - causes some hitherto hidden controls to be revealed
- In the "Authentication type" drop down-thing, I unchecked and re-checked Azure certificate
- Then the Save button magically became enabled!
Hopefully this will be fixed and my weird hack will not be needed for much longer :/