In the installation file (method.install.php line 115), the code was trying to
access a property that didn't exist:
$this->SetTemplate('mamsreg_emailuseredited_texttemplate',
$this->dflt_emailuseredited_texttemplate);

The Solution:
Added the missing property to the MAMSRegistration class:

var $dflt_emailuseredited_texttemplate = '
Greetings {$name} welcome to {sitename}. This email is being sent because,
although you have already registered with {sitename}, there was an error in your
input. The administrator has done his best to correct this data, and you are now
being sent an updated registration form. Please click on the following link and
enter your username, password, and the unique code below

Follow this link: {$url}
{if !empty($password)}Password: {$password}{/if}
Code: {$code}
';