I have recently upgraded a site from v6.1.6 to v7.10.4 (in multiple steps) and everything is fine with the exception of the Membership functionality.

The site has several member types and member groups already has 100+ members. However, now I cannot create a new member. When I do I get a NullReferenceException in the logs:

2018-05-15 11:23:08,894 [P6340/D9/T8] ERROR Umbraco.Web.Editors.MemberController - Unhandled controller exception occurred
System.NullReferenceException: Object reference not set to an instance of an object.
    at System.Web.Security.Roles.GetRolesForUser(String username)
    at Umbraco.Web.Editors.MemberController.PostSave(MemberSave contentItem)

Looking at the MemberController code in the CMS source, I can see that GetRolesForUser() on line 302 is expecting a non-null contentItem.PersistedContent.Username property but this is null:

var currRoles = Roles.GetRolesForUser(contentItem.PersistedContent.Username);

I know that it is the Username property that is null since on a previous line MapPropertyValues() is called which references contentItem.PersistedContent and if this property was null, the exception would be thrown here and be shown in the stack trace.

Can anyone think why this would be the case?

Thanks.