US Bank account sync/update error.

Welcome to QuickBooks Community, cj369. Thank you for providing detailed information or for sharing different errors you've encountered while manually updating your bank account.

Usually, error 9999 is one of the script errors that are generated by the bank. They usually indicate that there's a problem with the connection that Intuit uses to download your banking information. With this, I recommend checking your bank's website. There may be an issue on their end and it loses connection with QBO every time there are updates. 

Here's how:

1. Go to the Banking menu at the left panel. Then, choose Link account or Add account. Note: You won’t add an account, you just need the listed URL.
2. Search for your bank and select it. Then, Copy the URL listed for your bank.
3. Sign in to your bank or credit card's website using the copied URL. If you can sign in, there isn't a major issue. 
4. Check your account for messages, notifications, or alerts and look for any display issues as you navigate the website. These can block QuickBooks from downloading new transactions.
5. Check your bank’s website for any announcements about new security requirements.

Once everything looks good, go back to the Baking page in QuickBooks Online. Then, click the Update button again. For additional information and to see further troubleshooting steps so you can see the latest transactions, you can refer to this article: What to do if you get a bank error or can't download transactions in QuickBooks Online.

If the issue persists, you can manually bring the transactions into QuickBooks Online using a WebConnect file. Download the statements from your bank's website. Please follow the steps below.

1. Sign in to your bank's website.
2. Follow your bank's instructions for how to download CSV files of your transactions to your computer. Every financial institution has different steps. Here are the steps for Bank of America, Chase, and Wells Fargo.
3. Check the date range for the download. It should be at least one day before the date of your oldest transaction.
4. Download your transactions. Save the file somewhere you can easily find it, like your computer's desktop.

You can get more details about this process in this article: Manually upload transactions into QuickBooks Online. This will provide you a link to know if you have the correct format as well as steps on how to upload the file in QuickBooks.

Once imported, I recommend editing, assigning, and categorizing your transactions. Also, you can match them to the existing entries in the software. Just go to the For Review tab from the Banking menu. Doing so will help you ensure the accuracy of your financial records.

On the other hand, the other error you received is a server and browser issue. I suggest consulting an IT expert to check the cause of the issue and how to resolve this.

Thank you for your patience and understanding. Feel free to leave a comment below if you have any other questions about the error 9999. I'll be happy to help you out.

Customer support says it is an issue that many people are having. They are working on and will let me know when it is resolved.