This is a issue that came up with Joomla 3.7 and is here to stay it seems.
Basically, up to Joomla 3.6.5, you could use SCLogin without any other login menu item and everything was just fine, With 3.7+, you need to always have a Joomla user login menu item (Can be hidden).
Solution is to create a hidden login menu item in your mainmenu, clear your caches and you will be good to go. I just ran through that gauntlet a few weeks ago.
See:
github.com/joomla/joomla-cms/issues/1573...suecomment-303337223
Actually, they are focussed on a different reason for the issue but the impact is the same and so is the solution.