Same issue here! ...SOLVED
Short answer: I edited mydomain/pathtoroundcube
/config/main.inc.php and changed
$rcmail_config['default_host'] = 'ssl://imap.site.com';
$rcmail_config['default_host'] = 'mail.mydomain.com
In fact, if you find the above code in main.inc.php there is a nice explanation of options for it just above the code, makes sense.
This is a website how-to for installing Roundcube that gave me the idea to do the fix just explained:https://my.hostmonster.com/cgi/help/265
So now I can login to my Roundcube mail (0.8 version),
but it looks like a different GUI skin
than the one i had seen when using my host service Roundcube email viewer which I am guessing is a 0.7 version. I really do NOT care for this new skin, I want that old skin back that was kind of all silver-white, not this new skin that has blues and is a waste of screen space. Update-- I found the User Interface settings in the new 0.8 roundcube GUI and was able to change the skin from the new default Larry to Classic, looks nice now with Classic. An aesthetic choice, no doubt some people will like Larry.