Hi Inga,
I had the exact same problem like you…test email goes through without issues, but sending invoice or report, error message. In my case it was an issue with email setting in AVG…it seems that scanning outbound emails (SMTP) was creating a problem. Once I deactivated that box, all emails are going through without any issue. If you have something similar in your antivirus program, try to deactivate it and see if that works. Hope this help 