This issue is caused by unsuccessful update from WordPress 3.1 to 3.2.x. WordPress 3.1 does not define MAX_MEMORY_LIMIT on "includes/default-constants.php". which is being called on wordpress 3.2's admin.php
error_log message: PHP Fatal error: Allowed memory size of 262144 bytes exhausted (tried to allocate 1853353 bytes) in....
resolution: manually update the file by downloading the latest copy of wordpress and replace the includes folder.
Notes: Before uploading "include" folder of the new version:
1. Rename your existing "include" folder to something else as a backup (e.g. include_bak)
2. Create a backup of your database:
mysqldump [databasename] -u[database_username] -p[db_user_password] > backupfilename.sql
After making the necessary backups, login on your admin panel. WordPress will now prompt to update your database after this fix so make sure you created a backup before proceeding.
You should be able to login on your admin panel after the database update.