You can perform basic troubleshooting to see if it's a browser issue or not. This way, you can clear the outdated file that causes a small problem when adding your accountant with QBO. Please open your QuickBooks via a private browser.
Here's how:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Shift + N
Microsoft Edge: Ctrl + Shift + P
Then, add your accountant again. If you're able to do so without any errors, you can switch back to the main browser to clear the cache. You can also try using other browsers to isolate browser-related issues.