That’s a very generic message. It means it tried to read an item from a collection, but the item wasn’t there. That collection could be any number of things, like menu items, business records, etc.

ArgumentOutOfRangeException Class (System) | Microsoft Docs

Whoever diagnoses this will likely need more information. What was the user doing when they saw this? Is it repeatable?

This is a sporadic issue when trying to open Schedule board ,so it will be difficult to diagnose. Thank you!