Does joomla normally allow you to access your /administrator control panel using http://www.mydomain.com/administrator or is access to admin only allowed at http://mydomain.com/administrator?
I ask if it's only permitted by using
mydomain.com/administrator
because this is the case in my situation. Successfull login ocurrs there, but when I add www to the domain the backend login page just refreshes upon submitting credentials.
I tried getting some support in the joomla forums on this one as I thought i'd finally tracked down someone with the same symptoms, but I managed to get a split topic and labeled a thread hijacker.
I'm not sure if this particular issue is template related or just joomla configuration related.
****Update****
Since my work environment is based on IE only I had only been testing with IE. After using FireFox I noticed that this issue goes away all together. So... Apparently an IE issue.