>>I'm not 100% certain but I think we had the same problem occasionally. The solution was a simple retry, I believe.
>>
>>I'll try to find out more details from my former colleague.
>
>This happens every time I run it. I have not tried this code on my machine before, but it works fine for other developers and in production.

Then it must be a different problem. In our case as I now recall it was about API caused an exception in MailMerge module.