After upgrading or reinstalling the Alert Plus Web Part, the page where it is located displays the following error at the top:
System.ArgumentException: The specified view is invalid. at Microsoft.SharePoint.SPViewCollection.get_Item(String strDisplayName) at Bamboo.WebParts.AlertPlus.g() at Bamboo.WebParts.AlertPlus.OnInit(EventArgs e)
Go to 'View All Site Content' in the problem site and delete the Alert+ Configuration List. This will remove the error from the page and the list will be recreated automatically afterward. Unfortunately, any alert items that were previously created will be lost in this process, so they will need to be recreated.