I want to know what the matter is with her.
I want to know what's the matter with her.
I want to know what's her problem.
Is "I want to know what's the matter with her" and 'what's the matter' incorrect as Cambridge dictionary suggests (http://dictionary.cambridge.org/grammar/british-grammar/matter). In their explanation it is suggested that the word 'matter' be replaced with 'problem' as is written in the 3rd example.
Thanks for whatever help you can provide on this.
I've read Rathony's link and agree that his link answered the question about inversion. Now my question is specifically about the correctness of using this wording and if it is correct is there a rule in its usage.