you for reaching out on the community and telling us about your issue. I've read over the thread and seen you've tried the steps that UKR has recommended. Since you're still getting the error I would recommend contacting quicken support so we can look over the log files.
I will also list our hows down below so you can find a time that works best for you.
Quicken Phone Support Current Hours
Monday through Friday
5:00 am to 5:00 pm PDT
Quicken Chat Support Current Hours
Monday through Sunday
5:00 am to 5:00pm PDT
In case of CC-501, 502, 503, 506 errors from a single bank
deactivate all accounts with
this bank using the same bank logon ID, including any old, long closed, hidden
and forgotten accounts that might still be activated. If you miss one, the
aggregation server involved in handling Express Web Connect - connected
accounts will not start with a clean slate and you will continue to have
problems. When done deactivating, restart Quicken and reactivate accounts.