Yesterday I was working with XSL transformations just fine... we updated to 1.0.9110.4 later yesterday and now when I go to the Manage Alert Details XSL Transformation in Central Admin, I get a page with "Unknown Error."
In the event log I found:
Exception Type: System.Web.HttpException Exception Message: Could not load type '"SPSolutions.SharePoint.AlertManager.Administration.ApplicationPages.ManageAlertTemplateAlertDetailsTransformGlobalAdminPage'.
That did it. Thanks.