I first found that my Account Customer ID of many years (an email address with "@" in it) was not acceptable inside Quickbooks.
So, I changed the Account Customer ID with Citi Cards and, one time at least, I was able to change it in Quickbooks Bank Feed Settings.
Now it's back to the old one in Quickbooks and I can't change it.
Yet, when I try to deal with the account bank feed I still get the message:
"Invalid Customer ID. Customer ID can only contain alpha numeric values"
Even trying to import a file of transactions from Citi hits this problem so I'm stuck now.