When working in Export Definitions it's possible to set filters and edit how these filters work for most fields that are added to the export definition. In this case users find that the 'Edit filter' option is grayed out or does not correlate to specific fields that they're trying to edit. This can occur when separating fields that exist under the same node into different positions in the export field list.
If users add 'Interaction > Date' as an export field, add a few other different fields such as 'Constituent > Lookup ID', and then go back and add 'Interaction > Comment' later down the list as an export field, they find that when attempting to edit the Interaction Date field that it instead attempts to edit the Comment field information making the Date field not valid for edit.
This scenario is resolved in version 3.0 of CRM. To resolve please update to the latest version.
Alternative Solution if running v2.94: This only occurs in Web Shell and does not occur in Click Once so an alternative solution is to log into the environment via Click Once to edit or make any necessary changes to the export definition. If there are any questions please contact support.