I have read 100 pages of this thread before I clicked what my error was. Amongst the stuff I read I saw something about a login loop, maybe if you do a search for that term and other combination youll hit upon the solution, it is here Im sure.
Best of luck

P.S. Also right when I started with zen cart I had a login loop issue due to shared SSL, I dont bother with that I just paid £50 for a dedicated SSL certificate - worth every penny!