You might try deactivating then reactivating online services for the
account. If that works, be careful because it might download a bunch of
tranactions you already cleared.
If that doesn't work, you can try calling the FI's customer services and
hope and pray they have someone who knows about Quicken,
When you are in the account register, go to the Edit Account Details
panel. Either by selecting it from the Options menu (the little gear icon
in the upper right corner) or by pressing Ctrl+Shift+E
The click on the Online Services Tab, then click on the Deactivate
button, It will deactivate online services for the account. Then click
Activate. It will ask for your userId & password and try to connect to
the financial institution. Assuming that works you should be OK.
In most cases Quicken uses the same userId and password that you use when
logging on to the FIs web site, or from their mobile app. But sometimes
they might be different. The only way to find out is to ask the FI's
customer support. Some are really good, others, not so much.