Joe,
I've added a tracker issue for us to investigate External URLs more and see what's going wrong. Something you can do in the meantime is edit the /components/com_jlinked/models/loginregister.php file. At line 69, you'll see the getLoginRedirect function. There are a few if statements in there. At the end of the block you want, simply change the $redirect variable to:
$redirect = JRoute::_('index.php?option=com_community&view=profile&task=dashboard', false);
The first block is for new user registrations. The second block is for logins. The third block is if you have JLinked configured to leave the user on the same page they logged in on (not what you have, it seems).
Again, we'll try to figure out what's going wrong for the next release, but hard-coding what you want for now is probably the best choice.
Thanks,
Alex