The log indicates a ValidationException. Sadly in debug mode (or log files) we don't see the validation message itself. If you disable debug mode you should get a validation error that will be shown in the front-end.
Most likely you're trying to register again with an email or username that is now taken. If you want to test the verification email again (after you changed the SMTP settings for example) you will have to create a different user or delete that user via the database or an admin account.
