cannot login after 2.7 upgrade

i made the upgrade to v2.7, and now i cannot access the Admin control panel. at the login screen, it appears as though my username and password are correct, but it simply asks me to login again, as if to re-authenicate my account after a lengthy idle period. i attempted to reset my password via email twice, and also tried to reset the password on the database directly by editing the varchar(64) value. none of those attempts worked. i also tried to login on one of my editors’ accounts, with the same result.

any ideas??