Edit 3: If I add a new fee (after it has been saved) and instead click on 'save and done', this is th emessage:
Fatal error: Uncaught CRM_Core_Exception: [0: Transaction integrity error: Expected to find active frame thrown in /home/rldb/www/www/sites/all/modules/civicrm/Civi/Core/Transaction/Manager.php on line 107