Annoying. This is the type of situations which I would prefer to examine hands on.
From the Immediate Pane, you can type: ? rst.Updatable. But come to think of it, the error would occur on rst.AddNew, not rst.Update.
For one reason or another, the update is rejected. Are there validation rules for the table, required fields, something like that? If you fill the same table elsewhere, do you use a different ID number? different dates? It could be a key validation. Who knows if the message is so cryptic.
(°v°)