Hi, Phil,
I'm thinking that you are trying to re-email it from the Email Queue and that this is where you are receiving the error.
If the invoice were still in the current batch from which you originally emailed it, you wouldn't receive the error. However, if the batch has since been posted, you will as it is trying to re-email it from the batch that no longer exists. This is how it has always worked; you probably just haven't encountered it before.
What I'd do is go to History and re-email the invoice from there.
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Regards,
Softrak Tech Support