With the RadAjaxPanel has stopped working I assume that updated content inside is no longer received upon some action performed by the user. If the postback does not originale from within this RadAjaxPanel it should be expected to retain its "old" content. If this wasn't the case before then you have had it nested in another RadAjaxPanel, asp UpdatePanel or RadAjaxManager setting and this was causing it to receive new content.
What I can suggest is using a regular ASP:UpdatePanel inside the RadToolTip and call its Update() method on the server when needed (in case you have set its UpdateMode to Conditional, which is the default setting for a RadAjaxPanel). If you use a regular ASP:UpdatePanel and leave the default UpdateMode which is Always it will be updated with each partial postback.
the Telerik team
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