I got the global configuration and media manager to work by deleting: libraries/joomla/session/storage/wincache.php
I compared your site to a clean 1.5.26 install and wincache.php is not in the core, however it is in the Joomla 2.5.4 core. So my guess is that at some point you mixed Joomla 2.5 with 1.5 files (which shouldn't be done).
If you've mixed files from the two versions, the easiest way to fix it is to install J2.5 in a new directory and add all your modules and plugins in. Then change the database connection in configuration.php to point to your original data base and see if it's all working. If it works, move the original install and replace it with the new one.
You could spend many hours comparing directories and trying to track down 1.5 files in a 2.5 install. So a clean install connected to the original database usually works.
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Unable To Access User Manager Are of my Joomla 1.5.24 Site 11 months 1 week ago #48723