I made an upgrade from Magento 1.4.2 to 18.104.22.168 and everything worked fine.
My frontend and backend worked properly. I can log in into the admin panel without issues until a few hours later when I clicked on log out from the admin panel, I am not redirected to the admin login page and instead, I get the following error.
Fatal error: Call to a member function getUsername() on null in C:\MAMP\htdocs\magento\app\design\adminhtml\default\default\template\page\header.phtml on line 33
I cannot show the admin login page anymore and I cannot log in into the admin panel. The frontend works fine. My guess is that the admin session did not close properly, I tried to clean the var/cache + var/locks + var/session + cookies from my browser + try different browsers => did not work
I tried every solution suggested in this topic, but nothing worked : Magento 1.9 Can’t login to admin panel!
For information, this is a local environment. All the third party modules are disabled and I removed all local devs. I also tried an upgrade from 1.4.2 to 22.214.171.124, the upgrade worked fine, frontend and backend are working and I get the exact same issue in the admin after a few hours (I was logged in the admin but could not log out / re-login).
I also have a local installation of Magento 1.4.2 and here everything works fine I can log in/ log out / re-login from the admin without any problem.
Do you know how I can solve this issue?