I reported the following issue to Quicken support who opened case number 10887148 after verifying the issue:
My Quicken environment is Quicken Classic Deluxe for Windows, Version R55.15, Build 27.1.55.15. I have four accounts connected via EWC to Leaders Credit Union - TN. They update fine. However, I opened a fifth account two months ago. I have been trying to add this account via EWC with no success receiving a CC-501 error. I contacted Quicken support who verified the issue and opened case number 10887148. The support tech said this should be resolved in 48 hours. Well, it's been a week with no resolution.
I have no issues with adding accounts from my other financial institutions.