This is a really peculiar issue that we are not familiar with. Since I was unable to reproduce this behavior I can only be guessing, but please make sure that the scope of the manager's object is correct, i.e. should the manager you cannot access be accessible from the point you are trying to use it (or is it in a parent page, or in a user control, etc)?
You can also try using the FindControl() method to get a reference to the proper server control.
If the above suggestions do not work for you please open a support ticket and send us a simple runnable project that displays this behavior so we can investigate it further.
the Telerik team