Loading old courses grading causes backend 500

[RichDom2185]

[sentry-io]

Sentry Issue: CADET-BACKEND-E08

DBConnection.ConnectionError: tcp recv: closed (the connection was closed by the pool, possibly due to a timeout or because the pool has been terminated)
  File "lib/ecto/adapters/sql.ex", line 1054, in Ecto.Adapters.SQL.raise_sql_call_error/1
  File "lib/ecto/adapters/sql.ex", line 952, in Ecto.Adapters.SQL.execute/6
  File "lib/ecto/repo/queryable.ex", line 232, in Ecto.Repo.Queryable.execute/4
  File "lib/ecto/repo/queryable.ex", line 19, in Ecto.Repo.Queryable.all/3
  File "lib/cadet/assessments/assessments.ex", line 1988, in Cadet.Assessments.submissions_by_grader_for_index/2
...
(3 additional frame(s) were not displayed)

[RichDom2185]

Seems to be an unhandled issue that only showed up after the Aug 17 deployment