Hi there, christofar-sarka.
Thank you for visiting the QuickBooks Community. I'll be sharing troubleshooting details to ensure that your added user accepts the invitation without any error.
Sometimes a cache full of history and temp files can cause unexpected behaviors, such as receiving an error message when accepting an invitation. With this, I recommend that your new user accept the email invitation using a private browser. This is to rule out the possibility of a webpage issue, and private browsing doesn't store local files or cache.
Use these keyboard shortcuts based on the browser you're using:
- For Firefox or Microsoft Edge, press Ctrl + Shift + P.
- For Chrome, press Ctrl + Shift + N.
- For Safari, hold down Command + Shift + N.
If it works in incognito, the user needs to clear the cache of the regular browser. This removes the history or log of sites so you can start with a clean slate. You can check this article to see the steps: Clear cache and cookies to fix issues when using QuickBooks Online.
If the steps above didn't work, use another supported browser as alternatives.
You may refer to this article to see details on how you can manage users' permissions and how to delete them if needed: Add and manage users in QuickBooks Online.
You can always post here if you have further questions about managing your users in QuickBooks. I'll be around to help you in any way I can. Take care always and have a good one.
