What would cause this OLE error? (packet with an invalid header)

I've written an application which prints labels using the BarTender .NET SDK from Seagull Scientific. The application runs perfectly day after day, printing thousands of labels. But then, after about 3 weeks, when I call the BarTender engine's Documents.Open method, I get this error:

OLE received a packet with an invalid header. (Exception from HRESULT: 0x80010111)

After this error my application limps along, managing to print a few labels, but must be manually restarted to restore full functionality.

I've asked about this on the Seagull Scientific user forum, but haven't gotten any help there yet. I'm hoping that someone who has run into this OLE error, prehaps in a context unrelated to BarTender, might shed some light here.