Let me share a bit of explanation about this, Mollcons.

Disconnecting and reconnecting allows us to establish a new and stable connection between your bank's website and QuickBooks Online. This new connection has a lesser chance of encountering the bank error 590 or other banking issues.

Then, you can add older transactions again if necessary.

If same thing happens after that, it'd be best if you contact our Customer Care Team just like MaryLandT suggested. They can review the details and provide you more information on what is happening.

You can always get back to us on how this goes so we can further assist you.