Safari on US Bank: "our system is not cooperating"

For the last few days when my wife entered https://onlinebanking.usbank.com into M1 Big Sur 10.4.2 and 10.5.1 Safari she immediately received the error message: 

Our system is not cooperating

Try connecting again 

She can access the US Bank site from a different user account using Safari on her machine. She can also get to it from from Chrome in her account. Edge didn't work at all -- it just hung.

I tried deleting Safari Cache and deleting all US Bank associated cookies/data. No change. I tried a Private window - no luck. I tried changing DNS provider - no help. I tried removing ALL Safari cache and cookie and other data - no change. I tried with our 1Password extension enabled and disabled.

So for now she's using Chrome. There's something wrong with either Safari or something deeper in her user profile but I don't know where to look next.