I can help access your QuickBooks Online (QBO) account successfully, @tsmithnhs.
As long as you have access to the account, you don't need to be the master admin just to log in to QBO successfully.
Since you're still having an error when trying to access your account using an incognito window, try to use other browsers like Google Chrome, Mozilla Firefox, Microsoft Edge, or Safari.
You can also try to click the I forgot my user ID or Password link in the sign-in page as a workaround. Not because you've forgot the password, but to refresh your account. Here's how:
- Go to this link and click the I forgot my user ID or Password link.
- Enter the email address that is associated with the account and click on Continue.
- Log in to your email provider and look for the reset email.
- Click the correct user ID or the Reset password button.
- Enter your current password. You can change it if necessary.
- Click Reset password.
Let me know if the reply section if you need further help logging-in to the account. I'm always here ready to help. Have a wonderful day!