There is a known issue with Dynamic Distribution Lists that target a very large volume of recipients whereby a DDL cannot be created or edited due to a timeout error. This error occurs due to the target user calculation that is timing out when retrieving the list from exchange online.
Latest update and guidance
This issue is being investigated under PRB0179707.
As a workaround any creation requests or updates to existing DDL’s can be raised as a request with the NHSmail Helpdesk (email@example.com).