We did use a unc path, and the same path with the same credentials are working when we use a different emailtask.
We also recreated the task and the credentials and the notification, but the problem stays.
The weird part is that the test button did work.
Maybe it's because the job is also running with the same credentials?
Anyhow, it's working with a 2nd task, so it's not really a problem at the moment.
We're using version 6.0.9, Build 17299