CC-501 error with Credit Union
TL;DR; check the Online Services tabs for errors for all accounts at the same institution.
Quicken doesn't inform you when it can't find some of your accounts at an institution. You have to stumble upon the error in the Online Services tab for that account.
I had the CC-501 error with my credit union using Express Web Connect. My CD accounts had matured, cashed out, and closed. I'm positive I had deactivated those accounts via the account edit Online Services tab. For 2 weeks I tried adding the new accounts via both the Add Account and the Online Services tab and kept receiving the CC-501. Meanwhile, my other accounts at the CU kept connecting and downloading just fine. Eventually, I went to edit one of the closed accounts and noticed on the Online Services tab that the closed accounts still had Online Services enabled but in an error state. After deactivating Online Services for those closed accounts, I was finally able to connect.