I have Quicken Classic for Mac and cannot get it to connect to any outside accounts. My bank account says it is linked, but doesn't show up in the sidebar as linked. Get error HTTP-400 when I try to add my credit card accounts (Chase and Citi).
In other, similar discussions, it is recommended that you try using a different browser. The issue may be caused by a browser plug-in or setting, using a different browser will often fix it.
My quicken is a desktop version.
If you haven't done so already, I'd say it's time to contact Quicken Support on the phone during posted hours of operation and request help. Let them do a screen share session with you to review the situation on your computer, to determine the nature of the problem and to figure out how to fix it.