Any login just goes back to login screen.
 

I am building our test site - finally got the db uploaded and set, everything "looks" ok, only I can't get into the actual admin CP - it logs me in then goes right back to the login screen. In the "blank screen" troubleshooting doc, I tried most of those solutions, and still nothing. Right now I am re-installing everything for the bajillionth time (learning experience!), but any thoughts what else it could be preventing me from getting in?