This is not the kind of service we want you to experience, @fwarden.
Are you using Bank of America? If so, there is an ongoing investigation where users getting error 590 when connecting with the bank. Rest assured that our engineer's team is working hard to resolve this issue permanently.
Though, the error 590 typically occurs if your Financial Institution updated their bank's website. To isolate this, let's perform some troubleshooting to verify if the error continues.
Let me show you how:
On the left pane, click Banking.
Click on the Update button. QBO tries to connect to your bank to manually update your account.
Tap the pencil icon next to the bank name and select Edit sign-in info.
Enter your user ID and password.
Hit Save and connect once done.
If the error persists, I recommend contacting our support as they have the tools to pull up your account and fixed the error.