I had been getting a cc-501 error trying to download my Citibank accounts for the past week.
Finally, I called Quicken support.
They said that they have made some changes in August to their servers and it impacted many bank downloads.
I don't know why August changes didn't cause a problem for me until October. Maybe they have been trying to fix the August errors and have now created more problems.
At any rate, they asked for my Connlog and OFX log files and will pass that to their tech team.
They said it would take a week to expect results.
Here is another annoyance:
I went to their support page and followed the path that said they would call me about my problem within 20 minutes.
Instead of calling, they sent me an email with an incident number and asked me to call them. They did not even include their phone number in the email. I had to go back to the web site to get the phone number.