I am utterly baffled by this situation. Someone who wants to register as a user on my site has written to me saying that they received the welcome email, but could not activate. The welcome email looks perfectly normal. I have tried using the activation link, and can confirm that the result is "Code not correct - please log in to send a new link".
When I tried this, the screen went immediately from the registration form to the list of questions, with no acknowledgement of the account registration. However, the welcome email was again sent.
The "please log in" instruction seems a bit odd, since the user cannot log in prior to activating the account. Indeed, I have retried the account creation, and attempting to log in with the email just submitted, and get the result "User not found".
There is no new entry in the database for this new user, nor is there any existing entry in the user table with either the email address or the user name just entered.
Whatever could be happening?