See the screen shot below. I was experimenting with documents attached to products. I attached the standard Vaccination Certificate to Vaccination - Tetanus.  I changed the doc template to specify Print Mode=Immediate.  Then I created an invoice and added the Tetanus shot and then clicked OK. The failure occurred.  This is on the demo system which has no printers - and this may be the cause.

Customer is J Bourke, patient is Muffett.

I won't disturb this setup - ie I will go and play with another customer and a different product and template.

I am just about to press the Send Error Report button, so you might want to check that the expected notification arrives.

Regards, Tim G