I am trying to reproduce your issue and need some more information. Have you set up a custom verification email response? Is the user verified after you click the link (you should be able to see this in the user details in your dashboard)?
no the verification e-mail has been reset to default, but we use our own SMTP server to send e-mails, thought emails are being generated at auth0.
And yes the users get verified using Database Authentication but that process runs fine, the strange bit is that while getting this error, the email gets verified and you get an activation success email after.
After some research, I’ve found instances of this problem occurring when an email client will automatically click user’s links for them. Specifically, Office 365 has been known to have this behavior. Another possibility is an email antivirus doing something similar with the link provided. Does your user’s email get verified before you actually click the link? This would be a symptom of this problem.
If you have a development tenant available you may want to try to see if you can reproduce the issue when using a mail provider that does not actually deliver the email to the end-user inbox (for example, https://mailtrap.io/).
Hope this helps! Let me know if there is anything else I can do to help.