WP 2.5 widgets: Fatal error

Sorry for starting yet another WP 2.5/widgets thread, but I didn’t find the exact same issue posted here before:

When I go to ‘Design’ >> ‘Widgets’ I cannot see any widgets if I select ‘Show all widgets’ or ‘Show unused widgets’. This means I cannot add new widgets to the sidebar. Instead, the following error message comes up:

Fatal error: Call to undefined function: tla_ads() in /MY_URL/wp-content/plugins/execphp.php(24) : eval()’d code on line 1