When a student want open a dialogue, he became the following error message:
Error: There is no one available to have a dialogue with

This error comes if a teacher exist in a course, the settings are correct. Please see this topic in the forum:
http://moodle.org/mod/forum/discuss.php?d=78228

I mean i found something, please can you confirm:
In the file /moodle/mod/dialogue/loacallib.php in line 219, here is what the line is showing at the moment:

if ($otheruser->authority == 0) {

I mean it must be looks:
if ($otheruser->id == 0) {

This works for me!
Andy