400 Reuqest Header of Cookie Too Large

This means that the browser has accumulated so many cookies for Intuit domains that the request header exceeds the server's size limit, and the login gets rejected before it even starts.

To fix this, perform a targeted cookie clear specifically for Intuit and QuickBooks domains rather than a full cache wipe, so your saved credentials for other sites remain unaffected.

Here’s how:

1. In Chrome, go to Settings > Privacy and Security > Cookies and other site data > See all cookies and site data.
2. Search for intuit and delete all results.
3. Do the same for QuickBooks.
4. Close and reopen the browser, then try logging in again.

For further guidance, check out this article: Clear cache and cookies to fix issues when using QuickBooks Online. 

If you have any additional questions, please don’t hesitate to reach out. We're always here to assist you.