Interesting. We have something similar happening at one of our clients, who are on 8.1. They have three companies, and occasionally, when sending out invoices by e-mail, a PDF for a different invoice is also attached to the e-mail. This invoice can even be from a different company! I've not seen that a restart fixes this, or if it does, it's only a temporary fix as it's been happening for some time. I've not seen those errors in the log, but then I've not done a restore of their backup since upgrading to 8.1.