After updating from 1.4 -> 1.5 i also see an error box in the Settings -> Websites in the new field “eCommerce”. Also, every few clicks i need to relogin to my Piwik installation.
It seems that 1.5 is a complete mess 
… preparing last night’s backup to go back to 1.4 …
There is an error. Please report the message and full backtrace in the Piwik forums (please do a Search first as it might have been reported already!).
Notice: Undefined index: ecommerce in /var/www/vhosts/clubbi.de/subdom/piwik/tmp/templates_c/%%05^052^0528167D%%SitesManager.tpl.php on line 215
Backtrace -->
#0 Piwik_ErrorHandler(8, 1 => ‘Undefined index: ecommerce’, 2 => ‘/var/www/vhosts/clubbi.de/subdom/piwik/tmp/templates_c/%%05^052^0528167D%%SitesManager.tpl.php’, 3 => 215, 4 => array (‘this’ => Piwik_Smarty::__set_state(array(‘template_dir’ => array (0 => ‘/var/www/vhosts/clubbi.de/subdom/piwik/plugins’, 1 => ‘/var/www/vhosts/clubbi.de/subdom/piwik/themes/default’, 2 => ‘/var/www/vhosts/clubbi.de/subdom/piwik/themes’), ‘compile_dir’ => ‘/var/www/vhosts/clubbi.de/subdom/piwik/tmp/templates_c’, ‘config_dir’ => ‘configs’, ‘plugins_dir’ => array (0 => ‘/var/www/vhosts/clubbi.de/subdom/piwik/core/SmartyPlugins’, 1 => ‘/var/www/vhosts/clubbi.de/subdom/piwik/libs/Smarty/plugins’), ‘debugging’ => false, ‘error_reporting’ => ‘6143’, ‘debug_tpl’ => ‘’, ‘debugging_ctrl’ => ‘NONE’, ‘compile_check’ => true, ‘force_compile’ => false, ‘caching’ => 0, ‘cache_dir’ => ‘/var/www/vhosts/clubbi.de/subdom/piwik/tmp/cache’, ‘cache_lifetime’ => 3600, ‘cache_modified_check’ => false, ‘php_handling’ => 0, ‘se…#1 include(’/var/www/vhosts/clubbi.de/subdom/piwik/tmp/templates_c/%%05^052^0528167D%%SitesManager.tpl.php’) called at [/var/www/vhosts/clubbi.de/subdom/piwik/libs/Smarty/Smarty.class.php:1263]#2 Smarty->fetch(’/var/www/vhosts/clubbi.de/subdom/piwik/plugins/SitesManager/templates/SitesManager.tpl’) called at [/var/www/vhosts/clubbi.de/subdom/piwik/core/View.php:138]#3 Piwik_View->render(array () called at [/var/www/vhosts/clubbi.de/subdom/piwik/plugins/SitesManager/Controller.php:58]#4 Piwik_SitesManager_Controller->index(array () called at [:]#5 call_user_func_array(array (0 => Piwik_SitesManager_Controller::__set_state(array(‘pluginName’ => ‘SitesManager’, ‘strDate’ => ‘2011-06-23’, ‘date’ => Piwik_Date::__set_state(array(‘timestamp’ => 1308838799, ‘timezone’ => ‘UTC’)), ‘idSite’ => 1, ‘site’ => Piwik_Site::__set_state(array(‘id’ => 1)))), 1 => ‘index’), 1 => array ()) called at [/var/www/vhosts/clubbi.de/subdom/piwik/core/FrontController.php:127]#6 Piwik_FrontController->dispatch(array () called at [/var/www/vhosts/clubbi.de/subdom/piwik/index.php:53]