Gradebook

I hope this is the right place to post this.
We are trying out Moodle with a small group of faculty here, and while it is generally well recieved, we've got gradebook issues that I can't seem to diagnose and fix.

First: This error message appears, often mutliple times, in the gradebook view: "Warning: Invalid argument supplied for foreach() in F:\web\moodle-test\moodle\mod\assignment\lib.php on line 1561"

Second: Instructors who set the student gradebook view to show letter grades find that what is shown to the students is invariably an "F"

Third: Most recently, we've had instances where grades entered by the instructor one day are gone the next.

Please help
Thanks,
Dave