"Cannot redeclare class CRM_Core_ClassLoader" in backup site
It sounds as though there is either a cached path to ClassLoader.php somewhere or there are two copies of the CiviCRM code under your modules directories. Various scenarios and solutions are discussed in this forum post: svn installation says "Cannot redeclare class CRM_Core_ClassLoader" Please post back here rather than on the forum, though.

If you're sure that there aren't multiple copies of CiviCRM under /home/mydomain/www/backup/ then the most likely fix is to clear the Drupal variable civicrm_class_loader, e.g. using drush:

drush vset civicrm_class_loader ''


There's a good chance the fatal error will prevent the use of drush. In that case, you can delete the row from Drupal's variable table where name = 'civicrm_class_loader'. It's possible the variables are cached in addition -- you may need to clear Drupal's cache tables manually. (A targeted approach -- in Drupal 6, anyway -- is to delete the row from Drupal's cache table where cid = 'variables'.)