Initially reported to me as "mnet users cannot view grades in remote courses".

To replicate: Set up 2 moodles, called "Local" (can be any version), and "Remote" (must be 1.9.x)
Create user "Student" on Local, and give them all necessary rights on moodles Local & Remote so that they can sso from Local to Remote.
Create a course called "Course" on Remote, and enrol Student in it.
Log in as Student on Local, and roam to Remote. View Course, and click "View Grades" in the course admin block.
Observe that the error "Incorrect Userid" is displayed.

The expected behaviour is that a page is displayed showing Student's grades for activities in Course.

URL in bar at time of error is <wwwroot>/grade/report/user/index.php?id=<courseid>