from the thread:
After looking over things a bit further, it seems the new plugin uses a diffrent Id in the table ofID to store the last used conversationID. The old plugin used ID 50, the new plugin uses Id 600. This pretty much means that anyone upgrading the plugin from 1.2.0 to 1.3.0 will break their archiving if they have messages previously archived by the old plugin. All you need to do is get the value of the old conversationId marker like so:
and then insert that into the new Id Type like so:
This worked for my installation, you should verify that the numbers match up for yours. the DB upgrade script should do this for you, but does not.