New user login error: must be greater than zero

This is not a Drupal Core string.

Look for contribution modules (especially those that can interact with the user login or register form).

Fuzzytek, I've got the same problem on almost all user related forms. So, after a lot module unchecking and form submiting, the error stopped as soon as I disabled the XML Sitemap module, that's still in a dev version.
People must wait or work on it.

I can confirm this bug. It is an addon related bug. With mine, it was due to the Weather Facti module. A good rule I guess is to check the known issues of modules you install before installing and check your add-user functionality after adding too many modules.

It's not even a message error XML Sitemap would print out.

Why do you think it's a bug of this project?

monsterweb reports that he had the problem using the Weather Facti module.
As XML Sitemap doesn't output an error like Must be greater than zero, maybe it's that module to cause the problem.

I can confirm this is the module which raises the error.
At line 255 of weatherfacti.module there is the following code:

  if ($edit['weatherfacti_days'] == "0" || $edit["weatherfacti_days"] == "") {
    form_set_error("weatherfacti_days", "Must be greater than zero.");
  }

The error is caused by a wrong implementation of hook_user() (see #360211: Wrong implementation of hook_user()). In short: the module doesn't add its own fields in the user form, but it then checks the value contained in a form field that it didn't even add.

I would suggest to investigate more before to report a bug. Who reports the bug should be sure of which module causes the bug; I would rather think twice before to say that the problem is causes by XML Sitemap just because the problem vanished when I disabled xmlsitemap. As it has been proved, that was not the case.

I am closing this issue since it's for a Drupal version that isn't supported.