After changing username in "Change Domain Name" field "ugroup" in etc/webmin/virtual-server/domains/xxxxxxx still points to the old group, which doesn't exist any more. Field "group" point to the correct group name
Virtualmin 3.93.gpl on FreeBSD 8.2
The "ugroup" field is used to indicate that the domain's Unix user has a different primary group from the domain's own group - so when the domain is renamed, the ugroup is not expected to change.
This mode is rarely used though, and can only be selected at domain creation time. Do you want the domain's Unix user to be using a different group?