There is a long running, unresolved issue that is causing some users to get a CC-501 error when attempting to set up a Express Web Connection with BMO. There is an Alert for this issue in the Community; unfortunately, when the Alert was created, it only referenced "Bank of Montreal", which is not the correct/common name for the financial institution. If you are getting the CC-501 error for BMO, I suggest you refer to, and bookmark this alert, which references "Bank of Montreal":
https://community.quicken.com/discussion/7899767/ongoing-12-28-21-bank-of-montreal-returns-cc-501-error#latestIt would be helpful if one of the moderators could update the Alert to reference BMO, which is the common name of the Financial Institution and the name most people would think to search on. See attached screenshot from BMO.