I had the same problem as the OP, and even reverting to the old version didn't solve the problem, I was still getting a 403 Forbidden message everytime I logged in.
So I started poking around with things, and it seems like something in the .htaccess file was added in this update is causing the 403 error. I removed all of the iThemes Security stuff from my .htaccess file, and then I was able to get version 4.1.5 working again. (which then added stuff to the .htaccess file, but nothing that broke the site)
Hopefully this helps the developers pinpoint the issue..
On a side note, my logs don't seem to work now, but I'm hoping that's just a byproduct of upgrading to the new version and then downgrading again.