Unable to go to administration pages (access denied)
I recommend you to do the following:


backup your database
truncate drupal cache tables using phpmyadmin ui or from mysql command line
truncate role_permission table using phpmyadmin ui or from the mysql command line
try to login using super administrator, the user with id 1 
if nothing change with you, try to install another drupal project and see if you get the same error messages; maybe you have an issue with your php version or configuration.