I added IsArchived = 1 to procedure "pmessage_archive", since this is the field that PMList.ascx.cs uses to filter its result set when determining whether or not a private message should appear in the Inbox or Archive tabs.

UPDATE [{databaseOwner}].[{objectQualifier}UserPMessage] SET [Flags] = ([Flags] | 4) , IsArchived = 1 WHERE UserPMessageID = @UserPMessageID AND IsArchived = 0

IsArchived is a Calculated field based on [Flags] -- your database was not converted correctly.