After upgrading to 2.6 and activating the WordPress Stats plugin I get a message that WordPress Stats needs an API key. So I go to the settings, put it in, and when I try to save the settings I get this message (some info stripped):
Warning: require_once(***/wp-includes/class-IXR.php) [function.require-once]: failed to open stream: No such file or directory in ***/wp-content/plugins/stats.php on line 344
Fatal error: require_once() [function.require]: Failed opening required '***/wp-includes/class-IXR.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /***/wp-content/plugins/stats.php on line 344
I checked. All the files are there - obviously because I was able to activate the plugin and to go to the settings. class-IXR.php file is all in smallcaps on my server, but I think thats not a problem.