I am having this same problem.
I have a DA install that will not allow me to login. I started a reseller backup earlier. When I came back, I entered my username and password. It just redirects me back to the login page.
DA has been running fine since February. I was able to login to it a few hours ago.
/var/log/directadmin/error.log is empty.
I have done chmod 1777 /home/tmp
I have also rebooted the server several times.
I have tried other usernames/passwords, none of them will work