I see we've already replied to your email about this, but just in case someone with a similar problem encounters this forum post:
The Server Notification Error email you received was informing you that although the buyer's payment was completed, and we processed that buyer's order successfully -- sent emails, issued download links, logged the order data -- your account or product settings also specified a Common Notification URL (in Seller Admin > Edit Preferences, which applies to all sales) and/or Product Notification URL (in the settings of a particular product, which only applies when that item is purchased) where we should transmit a copy of the order data for additional processing, and that URL failed to accept the data we tried transmitting to it.
This error would only affect whatever custom needs you meant to satisfy by having us transmit the order data to that URL once we were done processing the order normally -- such as adding the buyer to a mailing list, or Stamping a PDF download to deter file sharing, etc. -- or it could indicate you'd entered something in a Notification URL field by mistake, perhaps misunderstanding what that setting was for.
