Could not find Pear DB library, this is one of the errors which you can have after entering your old password, two times your new password and press on submit button. In this case the fix can be very easy. It is possible that you only have to install php-pear and php-db and that is, this will fix the problem. If not, then you will have to dig deeper, but don’t worry, search in internet, there are many other suggestions.
Other error can be Could not make database connection, also after try to change the password. In this case you will have to do a small change in one file of change sclpassword plugin. In Debian Linux and other distributions made on Debian architecture this file is here /usr/share/squirrelmail/plugins/change_sqlpass/config.php there normally you will find this code.
$csp_dsn = 'mysql://db_user:password@localhost/db_name';
Where db_user, password and db_name are your correct database user, password and database name for your mail server. This “mysql” shows the database backend used in PHP or “phptype” how you can see it in other explanations. Here you will have to change “mysql” to “mysqli” if you use MySQL version >= 4.1 or MariaDB. For MySQL version <= 4.0 no need of change. But if you use other kind of database here are the currently supported database backends.
I hope this short topic can be helpful for somebody.