We had a user that had not used Mantis for a while and had forgotten their password. So I went into administration/manage users and selected password reset.
When the user opens their email and clicks the link, they get:

APPLICATION ERROR #1900

The 'lost your password' feature is not available.

Please use the "Back" button in your web browser to return to the previous page. There you can correct whatever problems were identified in this error or select another action. You can also click an option from the menu bar to go directly to a new section.