I use Firefox under XP with not any blockers nor firewalls (except of XP-own). No matter which browser I use.
I start the example page, log in (message "Hi Administrator!", e.g. loging-in performed!?), click "administrator" and ... come to the login form page (isn't it useless afzer logging in?). There I can input login data again, and then nothing happens except of coming to the login form again.
Hmmm... Meanwhile there are 2 (!) different session entries in the session directory of PHP. Both of them are created after submitting the extra login form, and not after submitting the form at the example page.
One session (the first?) is empty (0 byte), the second contains the following:
session_id|s:32:"71096d1c6148f950adb8216888088a13";session_user_id|s:2:"62";session_username|s:5:"admin";session_usertype|s:19:"Super Administrator";session_gid|s:2:"25";session_logintime|i:1129314662;session_user_params|s:0:"";session_userstate|a:0:{}
Sorry, my English isn't perfect - I am German. Nevertheless, I hope I can help. :-)
Ralph
post@rstahl.de