According to your original question, you're using this PHP code in the very beginning of your questionnaire:
$id = readGET('encID');
I'd recommend to use a question "device and transmittes variables" instead of the
readGET(). You can specifiy that this question shall read the "GET variable" encID. Please make sure, that the question is placed in the first page of the questionnaire.
If your questionnaire is called with a
&encID=.... (see URL to the questionnaire about
?), than a value shall be stored that you can use as placeholder in your redirect. If that value is absent, something's wrong with the URL you use to start the questionnaire.
Without known the starting URL that you have used for your tests, I cannot identify the error for sure. Yet, issues with the initial URL (that must contain the respondent code) is not unlikeley. It's probably also the cause of the problems you encountered so far.
Let's clarify the redirect, as soon as the encID is visible in the data.