The issue stems from the content div element that has explicit background color. The loading sign is a background-image on the main tooltip wrapper, so having explicit height and/or scrolling causes the later element to hide the root element, thus, hiding the loading sign as well.

You can work around this by removing the explicit color declaration for all skins:

div.RadToolTip div.rtContent
{
	background-color: transparent;
}

The error that is thrown on page load is that two components with the same ID cannot be added to the page.

Possible workarounds:
1) set EnableAriaSupport to false
2) use client-side code to show tooltips, as shown in this demo: http://demos.telerik.com/aspnet-ajax/tooltip/examples/radtooltipmanagerclientapi/defaultcs.aspx .