This error could be thrown by community server if it finds users that aren't in the instance of MemberRoleProfileProvider.
See CommunityServer.Users AddMembershipDataToUser() as an example
UPDATE:
I Solved this problem for my case by noticing that the usernames are stored in two tables - cs_Users and aspnet_Users. Turns out somehow the username was DIFFERENT in each table. Manually updating so the names were the same fixed this problem.
Also, the user would left out of membership in the following line of the stored procedure cs_Membership_GetUsersByName:
INSERT INTO @tbUsers
SELECT UserId
FROM dbo.aspnet_Users ar, @tbNames t
WHERE LOWER(t.Name) = ar.LoweredUserName AND ar.ApplicationId = @ApplicationId
The @tbNames is a table of names comes from cs_Users(?) at some point and therefore the usernames didn't match and user was not inserted in to the result later on.
See also: http://dev.communityserver.com/forums/t/490899.aspx?PageIndex=2