It will not remain a mystery, Per Kristian! You have got your Diphyus male on a needle, and when (not if) in a few years (I hope) there is a revision of Diphyus available, you can put it under your microscope again.
At the moment there are lots of work done around the world with revision of Ichneumonid genera. And there can and will be changes in taxonomy when DNA sequencing and mitochondrial markers are studied for all genera. Lots of renaming then have to be done. That is one of the main reasons why it is so important to collect your Ichneumons and not just take a photo outdoors.
Just be patient, you are a young man!
I have not checked if this one is Diphyus or not. You may get an answer before late from an expert.
I have done just a little more research: May be Spilichneumon occisorius?