I am on Windows with Quicken Delux. Both are latest version. I just deactivated one of my accounts at Synchrony and after 2 tries it picked up all accounts. I then tried an update and got the C-501 error again.
@TRoss have you tried doing an "Update now" from the account register for each of the accounts? In the account register, click on the gear icon in the upper right, then select "Update now". If the accounts are set up with one FI, all the accounts will be updated. If not, you will need to go to each account register separately. If the error continues, try closing and restarting Quicken. If the error persists, wait a bit and then try again. After that if it still persists then I would recommend contacting Quicken Support - https://www.quicken.com/support#contact-support
