Errors in 2.5.26, 3.2.6, and 3.3.5
From Joomla! Documentation
In the Joomla! 2.5.26, 3.2.6, and 3.3.5, an error was introduced into the code which processes Joomla! updates in the update component which causes errors when attempting to update Joomla.
When attempting to update to the next release, an "Invalid login" message will display in a popup window.
What is the cause
A faulty conditional check would never validate as true which would allow the update code to process correctly.
How to fix
Update to Joomla! 2.5.27, 3.2.7, or 3.3.6.
Note: Due to this bug, updates from the affected versions must be performed via the Extension Manager. Please see the following pages for instructions based on your Joomla! version.
Upgrading from an existing 2.5 version Upgrading from an existing 3.x version