Please verify the files on the server (for the admin) are an exact copy of the Zen Cart 1.5.4 distribution's admin folder (no modifications, no missing files, no extra files, etc).

What is the value of "admin" -> "configuration" -> "sessions" -> "session write directory"? This is used when you have configured Zen Cart to not use the database for STORE_SESSIONS (in the configure.php).

Post both "/includes/configure.php" and "/your_admin_folder/includes/configure.php" (from the server, not local development). Please use code tags around each file's contents (press the # button in the editor when replying - desktop version of the site). EX: [CODE]contents of the file[/CODE]. (Obscure the DB user, DB password, and admin folder if listed)

Also post the contents of "/includes/local/configure.php" and "/your_admin_folder/includes/local/configure.php" from the server if present (again wrap with code tags).