Thanks for coming with us today, grahamemay.
The Error 350 occurs when the connection expires or there is an update from your bank. I'd also recommend reaching out to your bank to verify if they have an ongoing website maintenance.
Learn more about fixing Error 350 through this link: Fix Error 350 in QuickBooks Online.
Be sure to let me know how it goes. I want to make sure this gets resolved. Have a wonderful day ahead.