Hello,
I just have had the same issue. I have some details about steps I have done to get this :
I did not have any account.
I received invitation email with url : https://manage.auth0.com/invite?token=xxxxxxxx
I clicked link and was asked to sign up.
I signed up using GitHub. My GitHub account has the same email where I received the invitation to.
I then was redirected to reject or accept invitation.
I accepted.
I was redirected to https://manage.auth0.com/invite?token=
saying :
Something went wrong!
The invitation you required does not exist. Check the URL (there should be a hex key after /invite?token= and make sure you copied it in its entirety. Please re-check the email that was sent to you and try again.
I tried in a private window with cleared cookies and caches with the same result.