The important question really was from what version! But – no matter.
As your users log in and use the forum then this notice should disappear. It should really only get triggered once for each user. And it is harmless. On top of this an update due very soon will add some extra code to avoid this.
The upgrade process should have cleared the offending data but we have seen a small handful of users for which this did not happen and we are unsure why some and not everyone. In your case we really want the housekeeping tool to effect all users bit, in fact that is not so easy although we do have a task to do this.
If you are familiar with getting into the database then I can tell you which column in which table needs to be emptied. If not then I would seriously suggest you wait.
The only thing that bothers me is the suggestion from what you say that this is being displayed on the screen. If this is the case then you should contact your hosting support immediately and ask them to turn off error messages being displayed. They should NEVER be allowed to display and hosting companies should always set up accounts with this turned off. Very bad practice.