SPO 2013 redirect using url parameter doesn't work

Using same solution proposed here: Confirmation Page / Custom List

Url is included in a function that generates newpost.aspx in a pop up. It worked once but then stopped which is weird.

function showPopup() { var options = { 
url: 'https://xxxxx/Lists/Posts/NewPost.aspx?Source=https://xxxx/thankyou.aspx',
title: 'Submit A Recipe', 
allowMaximize: true, 
showClose: true, 
width: 625, 
height: 850
}; 
SP.SOD.execute('sp.ui.dialog.js', 'SP.UI.ModalDialog.showModalDialog', options); }; 

Called with:

<a href="javascript:showPopup()" class="btn btn-success">Submit</a> </div>