I appreciate you for performing the troubleshooting steps on your end, 

fffcpa. Please know that we've received similar cases or reports about getting an error accepting the client invite. Let me route you to the right support team to assist you further on this matter.

I recommend contacting our QuickBooks Support Team to be added to the affected users list, ensuring you receive email updates on the investigation's status and resolution. 

Here's how:

1. Sign in to your QuickBooks Online company.
2. Click the Help (?) icon.
3. On the Assistant tab, enter Talk to a human.
4. Enter a short description of your concern and press Enter.
5. Select Contact Us to connect with a live support agent.
6. Choose how you'll want to get in touch with the support.

• Start a chat with a support expert.
• Get a callback from the next available expert.

Please ensure to review their support hours to know when agents are available. This way, you can contact them at a time that is convenient for you.

In case you need steps on how to keep track of clients and update the list, you can read this article: Manage clients in QuickBooks Online Accountant.

Please feel free to leave a comment below if you have other concerns about accepting a client's invitation in QuickBooks. I'm always ready to help. Take care, fffcpa.