Thank you for these questions.
I am thinking about adding the AjaxManager to the master page instead of one manager for each page.
Is there any disadvantage of using a single RadAjaxManager performance wise? I have a common type for Masterpage so I don't have to use findcontrol.
I believe this is the best approach -- access the AJAX Manager through a property from the MasterPage. No, there are not any disadvantages so feel free to follow this approach.
RadAjaxManager and RadAjaxPanel at the same application
What are the known issues?
There are several scenarios having both panel and manager in the same application, which throw errors and are hard or even impossible to resolve over ASP.NET AJAX base. Therefore, we recommend to use RadAjaxPanel in simple implementations only and leave the manager-only approach for the more complex scenarios.
3) In addition, about RadCodeBlock:
What's the difference between using it and a regular asp:placeholder?
Very good question! Actually there are no differences for the time present -- you can use either the placeholder or RadCodeBlock. We may however include additional functionality in RadCodeBlock in future, which the placeholder will miss.
returns an error while performing ajax postback
RadAjaxManagerProxy has "codeless purpose" -- it should help developers AJAX-enabling their applications design-time only. This is the reason we have not added AjaxRequest events for the proxy for example -- if you need some code, you can get the master manager instance and use it instead.
I hope this information helps.
the Telerik team