It can be challenging when you can't accept the invite, HughB. I'm here to help you resolve this.

The invite link could be corrupted or expired. It's the reason why you've encountered an error when clicking it.

To rectify this, I recommend deleting the email. Then, ask the primary admin to remove you as a user and re-invite you to generate a new email. I suggest sharing this article for their guidance: Add and Manage Users.

Then, accept the invite using Outlook online (instead of on your desktop) or another webmail client. Alternatively, open the link on your mobile device.

Once done, update your desktop mail client to the newest version.

If the issue persists, open the link using a private window again. This way, we can determine whether this is a browser cache-related issue. The browser utilizes cached data to load web pages quickly. However, excessive cache can lead to problems that impact the appearance and functionality of your account.

Here are the keyboard shortcuts: 

• Safari: Command + Shift + N 
• Google Chrome: CTRL + Shift + N
• Mozilla Firefox or Microsoft Edge: CTRL + Shift + N

If you successfully accept the invite, clear the browser's cache. Doing so deletes all the data from your browser and optimizes its performance. Using other supported browsers also helps us verify if this is a browser-specific issue. 

Moreover, I've added an article to help you learn more about user permission to ensure you can perform the tasks assigned to you: User Roles and Access Rights in QuickBooks Online.

Please keep us posted if you need additional assistance with the whole process. My goal is to ensure this gets sorted out.