We've had the same issue for 2 weeks - here's what worked for us finally:

1. search for BMO in the search field at the top (don't just use the BMO option included in the default list). 

2. Try ALL the different BMO log in options. Yes, even the ones that clearly don't apply to you. In our case we have a BMO Business Credit card only (no debit card, no 'Client ID', no personal accounts) The log in that finally worked was the 'BMO Personal' - makes no sense, but I don't care. It re-established the connection.

best of luck if you're also struggling with this mess.