Error messages after changing theme

After changing and updating theme for my blog, I get the following errormessages everytime i try to save a new or edited message. They do get saved by the way, but still I’d like to fix these messages. Does anyone know what they mean and how to fix it? Thanks for helping out.

Warning: include_once(/web/sites/amen/12/131/47164/public/www/wp-includes/class-IXR.php) [function.include-once]: failed to open stream: No such file or directory in /web/sites/amen/12/131/47164/public/www/wp-includes/functions.php on line 880

Warning: include_once() [function.include]: Failed opening ‘/web/sites/amen/12/131/47164/public/www/wp-includes/class-IXR.php’ for inclusion (include_path=’.:’) in /web/sites/amen/12/131/47164/public/www/wp-includes/functions.php on line 880

Fatal error: Class ‘IXR_Client’ not found in /web/sites/amen/12/131/47164/public/www/wp-includes/functions.php on line 883