This is in the manager/includes/lang/english.inc.php file
The line is:
$_lang["weblink_message"] = 'A weblink is a reference to an object on the internet. This could be a document within MODx, a page on another site or an image or other file on the internet.<p>';
The extra <p> at the end of the line needs removing to keep the html valid. This is causing issues with the ManagerManager plugin when it tries to configure a weblink page.
Should be:
$_lang["weblink_message"] = 'A weblink is a reference to an object on the internet. This could be a document within MODx, a page on another site or an image or other file on the internet.';