Hi Ingo Oltmann,
Generally, Prometheus RadAjax and Prometheus RadToolTip work together. However, the current version of the suite introduces a bug in the following situation - RadAjaxManager that has in its AjaxSettings the ID of the RadToolTIpManager as an updated control, and you are using the AjaxUpdate event of the tooltip manager. In this case the callback to get the content of the tooltip that is about to open is replaced with a postback. In this case, we usually recommend to replace the RadAjaxManager with a RadAjaxPanel or an UpdatePanel.
However, I am not aware of a problem leading to the AjaxUpdate event not firing. If you could prepare and send us a simple running project, demonstrating the problem, we will have a look at it and will be able to provide additional information.
the Telerik team