Migration to databases Domain Users do not have access
Users are tied to logins with SIDS. If you moved your DBs, you may need to recreate that mapping.

You can either create another login for DOMAIN\DOMAIN USERS and go through and manually remap, or
run sp_revlogins (link) and sp_change_users_login 'Report' (link)

If sp_change_users_login 'Report' finds orphans, run sp_change_users_login 'Auto_Fix', 'user'.