Cannot edit user profiles

While trying to edit a user profile I get a blank page and this errors in my PHP log.

[17-Jul-2006 13:09:09] PHP Notice: Undefined index: rcmail_password in /Applications/MAMP/htdocs/drupal/modules/rcmail/rcmail.module on line 73
[17-Jul-2006 13:09:09] PHP Fatal error: Call to undefined function mcrypt_get_iv_size() in /Applications/MAMP/htdocs/drupal/modules/rcmail/rcmail.module on line 154

I'm using Drupal 4.7.2 and MAMP (Apache 2.0.55, MySQL 5.0.19 and either PHP 4.4.2 or PHP 5.1.2)
I can access roundcube from http://mysite.com/modules/rcmail/roundcube and it seems to work fine, but from the menu I also get a blank page but without any errors in the PHP log.

Any ideas would be greatly appreciated!!