The aforementioned issue has been observed while accessing Domino Archives using Windows Authentication (therefore using the url like 'https:///EnterpriseVault/Search/').

This happens if the format of the stored domino savesets is 'NATIVE' instead of the expected 'MUD'
This issue is resolved in 14.4.1 & later versions. Go to the Download Center and upgrade.
After the upgrade from a previous version (like 12.x) the items downloaded from Enterprise Vault Search (EVS) within Domino Archives appear to have an 'unknown file type' extension instead of the expected 'eml'.
JIRA: CFT-5329