Why is a string reference different to a string?

At least I think thats what my question is. Basically if I run my function like:

GSFU_Demo_Utils.RetrievePassword("simon",true);

it works fine.

but if I run my function like:

   string enteredUsername="simon";
    GSFU_Demo_Utils.RetrievePassword(enteredUsername,true);

the function fails.

How on earth can this be?

I would start by looking into the code behind this function and then if nothing becomes apparent then try contacting the developers of GSFU.