I also encounter the issue, but after upgrading WordPress to 4.6.1 and subsequently Sucuri to 1.8.1 – xcache settings were not changed. Do you have a workaround?
A “normal” login works fine for me with WP 3.4.2 and SL 2.4.
I am able to reproduce the problem if I go to my site, login, press the browser’s “back” button and try to login again (not having logged out before).
