I am badly in need of using f:ajax functionality for my application. I have a form in which I have to use ajax to submit that form and then update another component with this form data that got submitted. The component that is to be updated in my case is a fragment tag.
My code is as follows:
Hint: use the Code button to create tags that can be wrapped around your XML and make it easier to read.
I can't see anything wrong with your outline there, so the things I'd look for would be possible mis-spellings or mis-capitalizations of "fragmentid". Also, check to see if ui:fragment is an actual UI element. A good indication is to check its documentation and verify that the "rendered" attribute is indeed allowed for that element type.
An IDE is no substitute for an Intelligent Developer.