Thank you for bringing this to our attention, @lj8.
HSBC bank is currently experiencing multiple issues with open banking connectivity which includes the error message you're getting. Our engineers are now all hands working together to fix the problem as quickly as possible. At this time, you can have your client try connecting or re-authenticating again to resolve the problem.
Also, I encourage reaching out to our Support Team so a representative can add you or your client to the list of affected users in this investigation. Once added, you'll automatically receive an email notification with the latest updates. To contact support, check out this link: Contact the QuickBooks Online Customer Support team.
Get back to me if you have any other QuickBooks or banking questions. I'm a few clicks away to help. Have a good day!