Graph api returns 19999 error at 'List Assignments'

Greetings,

I am using Microsoft api beta(Education).

Among them, I'm using List Assignments, suddenly it returns 19999 error.

Code: 19999
Message: Something failed, the API cannot share any more information at the time of the request.

I refferd to https://docs.microsoft.com/ko-kr/graph/api/educationclass-list-assignments?view=graph-rest-beta.

Does anyone know why this is happening? );