If you haven't already, you should try to recreate the connection with your bank. You will need to disable your current connection and then recreate it using the New Connection button on the Online Banking Setup screen. To do so, you can follow these steps:
Select your bank account in Moneydance
Select Online --> Setup Online Banking
Click the "Disable" button. If you don't see this button, just close the window.
Repeat the previous steps for any additional accounts at the same institution.
Select Online --> Setup Online Banking (again)
Click the "New Connection" button and select your bank and hit OK
Continue with the setup process, entering your username/password
Make sure you pick "New Connection", even though your bank is still one of the connection choices after you disable it. Choosing your bank from the long list of available banks will force Moneydance to download the updated information, which should then allow you to connect.
Let me know if you continue to have connection problems after recreating your connection and we'll troubleshoot further.