It's a great pleasure to have your trust in posting your concern in the Community forum, Twisted. I'd be happy to give you the information about this error.
I know how crucial QuickBooks Online bank activities are to perform smoothly. A bank error 108 indicates that there is a message from your bank or credit card on their website, such as updates that could prevent the connection to QuickBooks. Here's how to isolate the issue:
Let's begin by reviewing messages on your bank or credit card's website.
Access the website for your bank or credit card.
Search for any notifications, messages, or updates.
The message should outline any procedures you need to do when you have to take action.
Then, update your bank connections to QuickBooks.
Log in to your QuickBooks Online account.
Go to the Bookkeeping menu, then select Transactions.
Click Bank transaction, then Update.
If you haven't noticed notifications on your bank's website and still encounter the error, I advise contacting your bank. They can provide you with details about certain types of messages that can cause error 108.