When adding a new email to user account on gitlab.com, gitlab sends an email with a link to verify the email address.
However, the URL is wrong and opening it gives a 404 error.
Example of wrong confirmation link contained in the received email :
https://gitlab.com/o/default/emails/confirmation?confirmation_token=xxxx
(In the above link I replaced the actual token with xxxx.)
Resending the token did not help.
After a few failed attempts, I supposed that it was a bug in gitlab and tried to fix the link manually. After looking at the URL for the profile, I finally found the correct URL :
https://gitlab.com/-/profile/emails/confirmation?confirmation_token=xxxx
Conclusion : it seems that there is a configurations error on gitlab.com that sends wrong endpoint for the email confirmation links.