Thank you for reaching out to the Community and telling us about your issue, though I apologize that you are experiencing this.
A CC-503 error indicates that you are entering invalid credentials. One way you can make sure that you are using the correct instance of TD is by using the link to the bank provided by Quicken in the Add Account box, as pictured below.
Click on that link, and if you are able to log in to the website then you should be able to use that instance to log in from Quicken. One thing to note is that TD Bank Web connect only offers Express Web Connect. TD Bank Online Banking - New, offers the Direct Connect option as well if you want/need that.
Do let us know if this helps or if you have any more questions or concerns.