Error 378 appears on 9 of our Chase accounts. Thousands of transactions are not being imported. We reconnected with Chase 100 times. Chase says connected. QB says 378.

I'm happy to see you here in the Community, @nev1.

Error 378 means QuickBooks no longer has a connection to all of the accounts under that financial institution login. This can happen if a user removed access to the accounts on the FI website. Being unable to select all the accounts that you wanted Intuit to have permission to in the OAuth window can also cause this issue. 

To fix this, we need to re-entitle the connection between QuickBooks and the bank.

Here's how:

1. Navigate to the Banking page.
2. Click on the account displaying the error 378.
3. Hit on the link that says “Sign in to [Bank Name]”
4. Manually enter bank login credentials
5. Select ALL ACCOUNTS needed to connect to Any QuickBooks Company.
6. Press on Next to continue.
7. Tick on Update to make sure transactions now download into the account.

If doing this doesn't make a difference, I recommend contacting our Customer Care Team. They have advanced tools that can help narrow down this matter. 

To reach them: 

1. Click on Help at the top menu bar.
2. Hit on the Contact Us button.
3. Enter a brief description of the issue in the What can we help you with? box.
4. Press on Let's talk.
5. Select on Get a callback.
6. Key in your contact details, then tap on Confirm my call.

I've also included here a link that contains some steps about fixing online banking errors for your reference: What to do if transactions are not downloaded? 

Please know I’m only a post away if you need anything else. I’ll be here to help.