I'm setting up Postfix Admin 2.91 and trying to use dovecot:SHA512-CRYPT for password hashing. In config.inc.php I have set:

// dovecot:CRYPT-METHOD = use dovecotpw -s 'CRYPT-METHOD'. Example: dovecot:CRAM-MD5
//   (WARNING: don't use dovecot:* methods that include the username in the hash - you won't be able to login to PostfixAdmin in this case)
$CONF['encrypt'] = 'dovecot:SHA512-CRYPT';

// If you use the dovecot encryption method: where is the dovecotpw binary located?
// for dovecot 1.x
// $CONF['dovecotpw'] = "/usr/sbin/dovecotpw";
// for dovecot 2.x (dovecot 2.0.0 - 2.0.7 is not supported!)
$CONF['dovecotpw'] = "/usr/sbin/doveadm pw";

I have also tried SHA256-CRYPT and MD5-CRYPT with same results (as I understand it, these do not include usernames in the hash)

In running setup.php, I get the following message when trying to create an admin account:

can't encrypt password with dovecotpw, see error log for details

Server error log reports:

1624#0: *6 FastCGI sent in stderr: "PHP message: dovecotpw password encryption failed.
PHP message: STDERR output: sh: 1: /usr/sbin/doveadm: not found" while reading response header from upstream <...> upstream: "fastcgi://unix:/var/run/php5-fpm.sock:" <...>

A couple quick checks:

# ll /usr/sbin/doveadm
-rwxr-xr-x 1 root root 423264 Feb 13 23:23 /usr/bin/doveadm*
# doveadm pw -l
CRYPT MD5 MD5-CRYPT SHA SHA1 SHA256 SHA512 SMD5 SSHA SSHA256 SSHA512 PLAIN CLEAR CLEARTEXT PLAIN-TRUNC CRAM-MD5 SCRAM-SHA-1 HMAC-MD5 DIGEST-MD5 PLAIN-MD4 PLAIN-MD5 LDAP-MD5 LANMAN NTLM OTP SKEY RPA SHA256-CRYPT SHA512-CRYPT
# doveadmin pw -s  SHA512-CRYPT
Enter new password:
Retype new password:
{SHA512-CRYPT}$6$<long string here>/

Using Dovecot 2.2, PHP 5.5, MariaDB 10, Postfix 2.11, nginx 1.6.0, Ubuntu 12.04.