Fatal error: Call to undefined function lock_initialize()
The function is called using code similar to the following, from Drupal.

  // Initialize the lock system.
  require_once DRUPAL_ROOT . '/' . variable_get('lock_inc', 'includes/lock.inc');
  lock_initialize();


_drupal_bootstrap_variables() is the only Drupal function that calls lock_initialize(). 

That code could fail if the file whose name is contained in lock_inc doesn't exist, but in that case I would expect another warning/error message, or it doesn't contain a function called lock_initialize().

I would check if there is code that sets $conf['lock_inc'], and which value is used. I would also check if there is code calling variable_set('lock_inc', ...).