I have roundcube set up for my domain on DH and it works great.
Your database connction stuff looks correct.
Try port 143 for IMAP connections, instead of 38. So:
$rcmail_config['default_port'] = 143;
For SMTP, try port 25. So,
$rcmail_config['smtp_port'] = 25;
Now, as long as
$rcmail_config['default_host'] = 'mail.krusnix.com';
Your users should be able to log into Roundcube simply by entering their mail user name and password. So,
USER = krusnix (instead of firstname.lastname@example.org)
PASS = [krusnix password]
Another possibility is that your DH user-name and mail-name may be different. For instance, I have a DH username 'support' that recieves email at the address 'email@example.com'. So, when I log into roundcube, I have to log in as 'support' not 'help', even though the address is 'firstname.lastname@example.org'.
You can check this by going to DH-Panel>Mail>Manage Email. Then, find the address in question. The mail-user login name is in bold on the second line.
For instance, the entry for my user is:
"Support Contact" support
17 MB / 200 MB (9%)
So, the mail-user login name is "support".
Hope this helps!