When opening a workspace, or trying to group by or filter by a column on a list, receive error, "List '<list_name_here>' processing error."
If received when opening a workspace, the workspace doesn't load. If received when filtering or grouping a specific column on a list, the group/filter never occurs.
This error sometimes occurs for reasons other than what is described in this Knowledge Article. This explains one possible reason and solution.
One of the field-columns present on the default list used in a workspace, or the list being sorted on, is being populated using a ForEachChild function. This is likely being done via business rule.
Either remove the column-field that uses ForEachChild from the list or remove the usage of ForEachChild for that field.