Symptoms (all related to the Gradebook):

1. The Course Total column indicates "Error" for all students. (see screenshot 1)
2. The User Report gives the following error message: "An error occurred during grade calculation: Probably circular reference or broken calculation formula." (see screenshot 2)
3. The export to Excel spreadsheet function gives the following error message: "An error occurred during grade calculation: Probably circular reference or broken calculation formula." (see screenshot 3)
4. There are no broken formulas in the Gradebook UI, meaning that there is no way to fix this error.

Steps to reproduce:

1. Create a manual grade item in the Gradebook.
2. Enter a formula in the grade item, referring to another grade item, such as =SUM([[X]],[[Y]]).
3. Then delete one of the grade items that are referred to in the aforementioned formula, e.g. delete the grade item referred to as "X".
4. Examine the formula; it should now have a ## token indicating that it is pointing to a non-existent grade item.
5. Edit the grade item and change the value of "Grade type" from "Value" to "None"
6. Examine the Gradebook - THE ITEM HAS NOW DISAPPEARED FROM THE GRADEBOOK GUI.
7. Examine the database (if possible) - the manual grade item still exists with gradetype=0 and the gradebook error persists. It is now only possible to solve this problem directly in the database.

Cause:

I have updated this report after our IT Support were able to track down the source of the problem.

This problem is caused by the behaviour under item 6 in "Steps to Reproduce" above; namely, SETTING THE "GRADE TYPE" OF A MANUAL GRADE ITEM TO "NONE" REMOVES IT FROM THE GRADEBOOK GUI.

This has happened on two separate courses which caused us major problems and it has taken us 6 months to track it down.