"Error: "Unable to add group ACL to 'CN=Object,OU=Notes Contacts,DC=Domain,DC=com' for member 'Us (4249124)

When creating groups, errors like the following appear in the log:

14:31:26 Processing Owners and Administators for object 'Object' (Object) [Object@domain.com]
14:31:26 DEBUG: Set Group Owner ACLs for Group [CN=Object,OU=Notes Contacts,DC=Domain,DC=com]
14:31:26 DEBUG: (323) CFGDB::GetRs: Enter
14:31:26 DEBUG: CFGDB::GetRs: Exit(0.0160257 ms.)
14:31:26 DEBUG: (2419) ADFWD::FindObject: Enter
14:31:26 DEBUG: ADFWD::FindObject - fullFilter: (|(proxyAddresses=SMTP:User@domain.com)(proxyAddresses=notes:User /Domino@Notes))
14:31:26 DEBUG: ADFWD::FindObject returns 1,dn="CN=User,OU=Notes Contacts,DC=Domain,DC=com",count=1
14:31:26 DEBUG: ADFWD::FindObject: Exit(6.98901 ms.)
14:31:26 DEBUG: (1067) ADFWD::GetObjType: Enter
14:31:26 DEBUG: ADFWD::GetObjType: Exit(0.0252623 ms.)
14:31:26 ERROR: [8810-117-25-00000000] Unable to add group ACL to 'CN=Object,OU=Notes Contacts,DC=Domain,DC=com' for member 'User@domain.com'
14:31:26 Valid ACL member could not be found

The object must be a mail or mail-box enabled user to set the owner of a group.